Fix anchor link showing next to visually hidden headings

[lazerg]

Description

• Closes Anchor for Footnotes becomes visible on hover, even though the Footnotes heading is not #4165
• Headings marked sr-only, like the “Footnotes” heading GitHub Flavored Markdown generates for a footnotes section, were still wrapped and given an anchor link. The heading stays invisible, so hovering that section showed a link icon sitting on its own.
• Anchor links are now skipped for visually hidden headings, in both the unified and Sätteri pipelines.

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🦋 Changeset detected

Latest commit: 0a31ae9

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

This PR includes changesets to release 8 packages

Name	Type
@astrojs/starlight	Patch
@e2e/basics	Patch
@e2e/custom-src-dir	Patch
@e2e/git	Patch
@e2e/legacy-collections-backwards-compat	Patch
@e2e/no-mdx-optimization	Patch
@e2e/ssr	Patch
starlight-docs	Patch

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