Support table of contents button for org-mode file previews
What does this MR do and why?
Closes #600361 (closed).
org-ruby doesn't generate heading anchors, so this MR introduces Banzai::Filter::MarkupHeadingAnchorFilter to inject them into rendered HTML.
Screenshots
Visually confirmed rendering on GDK-in-a-box.
Notes
- No docs updates included – this area isn't well covered by existing documentation. If there are specific docs that should be updated, please flag them.
- Backend engineer, but fairly new to Rails – feedback on Rails conventions welcome.
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