WIP: First class support for AsciiDoc

⚠ This MR is very WIP ⚠

See https://gitlab.com/gitlab-org/gitlab-ce/issues/30746#note_38067559 for more information.

TODO:

• Somehow fix HTML sanitization.
• Fill information in this description and add screenshots.
• Render callouts inside highlighted source code.
• …?

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What does this MR do?

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

#32635 (moved), #30746 (moved), #28823 (moved), #28724 (moved)

!13978

/cc @mojavelinux