Prevent heading anchor links on non-Starlight content

[HiDeoo]

Description

• Closes Markdown Headings not appearing correctly for non Starlight Content #3175

In Starlight 0.34.0, support for generating clickable anchor links for headings was added. For Markdown and MDX content, a rehype plugin is used to generate the links.

When the feature is not disabled, this plugin runs unconditionally, which means that these links are generated for non-Starlight content like individual Markdown page or content not from the Starlight docs content collection.

This PR prevents this behavior by checking if the virtual file path is part of the Starlight docs content collection first, and bails if it is not.

For testing, I relied on E2E tests as it seems that on Windows, file path separators are inconsistent when the rehype plugin is used on Markdown vs MDX files. I'll investigate and open a separate issue/PR for that on the Astro repo.

Here is a preview of the changes before and after the fix for an individual Markdown page:

Before	After

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🦋 Changeset detected

Latest commit: 9d798dd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Patch

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[Eveeifyeve]

LGTM, but I hate function in functions since the functions could be changed in the future, but it looks good.