Add cargo doc warning lint to github actions

[A-Walrus]

Describe your feature request

Currently running cargo doc --document-private-items on the master branch of helix results in 5 warnings:

warning: unresolved link to `AddedIndent`
   --> helix-core/src/indent.rs:436:40
    |
436 | /// Each of these nodes produces some [AddedIndent] for:
    |                                        ^^^^^^^^^^^ no item named `AddedIndent` in scope
    |
    = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `IndentResult`
   --> helix-core/src/indent.rs:444:10
    |
444 | /// The [IndentResult] is simply the sum of the [AddedIndent] for all lines.
    |          ^^^^^^^^^^^^ no item named `IndentResult` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `AddedIndent`
   --> helix-core/src/indent.rs:444:50
    |
444 | /// The [IndentResult] is simply the sum of the [AddedIndent] for all lines.
    |                                                  ^^^^^^^^^^^ no item named `AddedIndent` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unresolved link to `AddedIndent`
   --> helix-core/src/indent.rs:446:39
    |
446 | /// Specifying which line exactly an [AddedIndent] applies to is important because indents on the same line combine differently than inde...
    |                                       ^^^^^^^^^^^ no item named `AddedIndent` in scope
    |
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: `helix-core` (lib doc) generated 4 warnings
 Documenting helix-term v0.6.0 (/home/guy/Documents/Programming/Rust/Forks/helix/helix-term)
    Checking helix-term v0.6.0 (/home/guy/Documents/Programming/Rust/Forks/helix/helix-term)
warning: this URL is not a hyperlink
   --> helix-term/src/ui/prompt.rs:103:21
    |
103 |     /// Taken from: https://github.com/wez/wezterm/blob/e0b62d07ca9bf8ce69a61e30a3c20e7abc48ce7e/termwiz/src/lineedit/mod.rs#L516-L611
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://github.com/wez/wezterm/blob/e0b62d07ca9bf8ce69a61e30a3c20e7abc48ce7e/termwiz/src/lineedit/mod.rs#L516-L611>`
    |
    = note: `#[warn(rustdoc::bare_urls)]` on by default
    = note: bare URLs are not automatically turned into clickable links

warning: `helix-term` (lib doc) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 6.31s

One error is a URL not formatted as an automatic link, and the rest are due to out of date documentation referencing structs that don't exist anymore.
Having an automatic lint could help prevent this