Fix snippet heading anchors always scrolling to first file (for Markdown)

What does this MR do and why?

Resolves Heading anchors in a snippet always scroll to f... (#604237 - closed) for Markdown, following up to Add `header_slug_prefix` render option (gitlab-org/ruby/gems/gitlab-glfm-markdown!135 - merged).

Note

• Non-Markdown markup is already covered by Fix snippet heading anchors always scrolling to... (!242817 - merged).
• Repository file rendering is unaffected.

Manually verified in GDK-in-a-box; docs updated in doc/user/markdown.md.

Approach

Prefix heading slugs with the snippet's filename so that anchors no longer collide across files:

File (e.g.)	Before: Both scroll to first heading	After: Each scrolls to its own
planning.md	<h2 id="user-content-summary"><a href="#summary">	<h2 id="user-content-planning-summary"><a href="#planning-summary">
review.md	<h2 id="user-content-summary"><a href="#summary"> ← duplicate!	<h2 id="user-content-review-summary"><a href="#review-summary">

Changes

1. 9ee23a58 - bump: gitlab-glfm-markdown to pick up the new header_slug_prefix option
2. 48263784 - refactor: extract the slug generation logic previously used for non-Markdown into Gitlab::HeadingSlug, so Markdown can reuse it too
3. 667773f3 - feat: pass a filename-derived prefix as header_slug_prefix from GlfmMarkdown when use_filename_in_anchor is set

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