Make versions menu work on any web host, not just GitHub Pages

[goerz]

Replace the hardcoded GitHub Pages URL assumptions with an async discovery approach: walk up the current URL until versions.json is found (urlExists HEAD check). This removes the json_file and current_folder template variables from ext.py; github_project_url now defaults to None instead of auto-detecting from github.io URLs.

Keep getGithubProjectUrl(rootUrl) as a runtime JS function and use nullish coalescing so the conf.py setting takes precedence but github.io URLs still get auto-detected when the setting is null.

Add migration notes to HISTORY.rst.

Closes #25.

[Copilot]

Pull request overview

This PR updates the Sphinx-injected versions-menu JavaScript to no longer assume a GitHub Pages URL structure. Instead, it discovers the documentation “root” at runtime by walking up the current URL until it finds versions.json, enabling the menu to work on non-github.io hosts, and updates tests/docs accordingly.

Changes:

• Replaced GH-Pages-specific URL logic with an async runtime discovery (versions.json search upwards from current URL).
• Removed json_file and current_folder template variables and adjusted tests/custom templates.
• Updated README/docs and added migration notes in HISTORY.rst.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/docs_versions_menu/_template/docs-versions-menu.js_t	Implements runtime root discovery and preserves GitHub URL auto-detection as a fallback.
src/docs_versions_menu/ext.py	Removes hardcoded template variables and changes default github_project_url behavior.
tests/test_extension.py	Updates expectations for the new template outputs.
tests/test_extension/roots/test-custom/_templates/doctr-versions-menu.js_t	Updates legacy custom template used in tests to remove deleted template variables.
README.rst	Documents the new auto-discovery behavior.
docs/sphinx_extension.rst	Updates extension docs to match the new hosting assumptions.
HISTORY.rst	Adds migration notes for removed template variables and new behavior.

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