Reference: #362

To reproduce, copied an existing test case and ensured the test script included multiple `exec tfplugindocs migrate` invocations. This generated a test failure matching the bug report:

```
--- FAIL: Test_SchemaJson_MigrateAcceptanceTests (0.00s)
    --- FAIL: Test_SchemaJson_MigrateAcceptanceTests/time_provider_multiple_runs (0.32s)
        testscript.go:558: # Copyright (c) HashiCorp, Inc.
            # SPDX-License-Identifier: MPL-2.0
            # Multiple runs of tfplugindocs -migrate to verify idempotency (0.000s)
            # Run migrate command (0.301s)
            # Check template files (0.000s)
            > cmpenv templates/index.md.tmpl exp-templates/index.md.tmpl
            diff templates/index.md.tmpl exp-templates/index.md.tmpl
            --- templates/index.md.tmpl
            +++ exp-templates/index.md.tmpl
            @@ -31,36 +31,3 @@
             `triggers` are *not* treated as sensitive attributes; a value used for `triggers` will be displayed in Terraform UI output as plaintext.

             To force a these actions to reoccur without updating `triggers`, the [`terraform taint` command](https://www.terraform.io/docs/commands/taint.html) can be used to produce the action on the next run.
            ----
            -page_title: "Provider: Time"
            -description: |-
            -  The time provider is used to interact with time-based resources.
            ----
            -
            -{{/* This template serves as a starting point for documentation generation, and can be customized with hardcoded values and/or doc gen templates.
            -
            -For example, the {{ .SchemaMarkdown }} template can be used to replace manual schema documentation if descriptions of schema attributes are added in the provider source code. */ -}}
            -
            -# Time Provider
            -
            -The time provider is used to interact with time-based resources. The provider itself has no configuration options.
            -
            -Use the navigation to the left to read about the available resources.
            -
            -## Resource "Triggers"
            -
            -Certain time resources, only perform actions during specific lifecycle actions:
            -
            -- `time_offset`: Saves base timestamp into Terraform state only when created.
            -- `time_sleep`: Sleeps when created and/or destroyed.
            -- `time_static`: Saves base timestamp into Terraform state only when created.
            -
            -These resources provide an optional map argument called `triggers` that can be populated with arbitrary key/value pairs. When the keys or values of this argument are updated, Terraform will re-perform the desired action, such as updating the base timestamp or sleeping again.
            -
            -For example:
            -
            -{{tffile "examples/example_1.tf"}}
            -
            -`triggers` are *not* treated as sensitive attributes; a value used for `triggers` will be displayed in Terraform UI output as plaintext.
            -
            -To force a these actions to reoccur without updating `triggers`, the [`terraform taint` command](https://www.terraform.io/docs/commands/taint.html) can be used to produce the action on the next run.

            FAIL: testdata/scripts/schema-json/migrate/time_provider_multiple_runs.txtar:13: templates/index.md.tmpl and exp-templates/index.md.tmpl differ
```

To fix this, changed the internal logic from opening the template file multiple times with `os.O_APPEND` to opening the template file once with only `os.O_WRONLY|os.O_CREATE` and passing the file handle around instead.

Reference: #335

Previously, the automatic `id` attribute handling intended for root level schema blocks was applied to all blocks, causing the generator to errantly set the grouping to `Read-Only` and the description to `The ID of this resource`. This was captured via a new integration test:

```
--- FAIL: Test_SchemaJson_GenerateAcceptanceTests (0.00s)
    --- FAIL: Test_SchemaJson_GenerateAcceptanceTests/nested_id_attribute (0.25s)
        testscript.go:558: # Copyright (c) HashiCorp, Inc.
            # SPDX-License-Identifier: MPL-2.0
            # Ensure only root level id attribute receives automatic description (0.236s)
            > [!unix] skip
            > exec tfplugindocs --provider-name=terraform-provider-scaffolding --providers-schema=schema.json
            [stdout]
            rendering website for provider "terraform-provider-scaffolding" (as "terraform-provider-scaffolding")
            exporting schema from JSON file
            getting provider schema
            generating missing templates
            generating missing resource content
            generating new template for "scaffolding_example"
            generating missing data source content
            generating missing function content
            generating missing provider content
            generating new template for "terraform-provider-scaffolding"
            rendering static website
            cleaning rendered website dir
            rendering templated website to static markdown
            rendering "index.md.tmpl"
            rendering "resources/example.md.tmpl"
            > cmp stdout expected-output.txt
            > cmp docs/index.md expected-index.md
            > cmp docs/resources/example.md expected-resource.md
            diff docs/resources/example.md expected-resource.md
            --- docs/resources/example.md
            +++ expected-resource.md
            @@ -48,17 +48,17 @@
             <a id="nestedblock--list_nested_block_optional_id"></a>
             ### Nested Schema for `list_nested_block_optional_id`

            -Read-Only:
            -
            -- `id` (String) The ID of this resource.
            +Optional:
            +
            +- `id` (String)

             <a id="nestedblock--list_nested_block_required_id"></a>
             ### Nested Schema for `list_nested_block_required_id`

            -Read-Only:
            -
            -- `id` (String) The ID of this resource.
            +Required:
            +
            +- `id` (String)

             <a id="nestedatt--optional_object_attribute"></a>
            @@ -72,33 +72,33 @@
             <a id="nestedblock--set_nested_block_optional_id"></a>
             ### Nested Schema for `set_nested_block_optional_id`

            -Read-Only:
            -
            -- `id` (String) The ID of this resource.
            +Optional:
            +
            +- `id` (String)

             <a id="nestedblock--set_nested_block_required_id"></a>
             ### Nested Schema for `set_nested_block_required_id`

            -Read-Only:
            -
            -- `id` (String) The ID of this resource.
            +Required:
            +
            +- `id` (String)

             <a id="nestedblock--single_nested_block_optional_id"></a>
             ### Nested Schema for `single_nested_block_optional_id`

            -Read-Only:
            -
            -- `id` (String) The ID of this resource.
            +Optional:
            +
            +- `id` (String)

             <a id="nestedblock--single_nested_block_required_id"></a>
             ### Nested Schema for `single_nested_block_required_id`

            -Read-Only:
            -
            -- `id` (String) The ID of this resource.
            +Required:
            +
            +- `id` (String)

             <a id="nestedatt--computed_object_attribute"></a>
            @@ -114,7 +114,7 @@

             Read-Only:

            -- `id` (String) The ID of this resource.
            +- `id` (String)

             <a id="nestedblock--set_nested_block_computed_id"></a>
            @@ -122,7 +122,7 @@

             Read-Only:

            -- `id` (String) The ID of this resource.
            +- `id` (String)

             <a id="nestedblock--single_nested_block_computed_id"></a>
            @@ -130,4 +130,4 @@

             Read-Only:

            -- `id` (String) The ID of this resource.
            +- `id` (String)

            FAIL: testdata/scripts/schema-json/generate/nested_id_attribute.txtar:9: docs/resources/example.md and expected-resource.md differ
```

The schema rendering logic could likely use a full refactoring as it has very high code complexity, but this change is only a very targeted fix for the acute and errant `id` attribute handling behavior.

Bumps [github.com/yuin/goldmark](https://github.com/yuin/goldmark) from 1.7.0 to 1.7.1.
- [Release notes](https://github.com/yuin/goldmark/releases)
- [Commits](yuin/goldmark@v1.7.0...v1.7.1)

---
updated-dependencies:
- dependency-name: github.com/yuin/goldmark
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>