Resolve "Expand API: Render an arbitrary Markdown document"

What does this MR do?

Adds /api/v4/markdown API endpoint to render markdown text.

Parameter	Type	Required	Description
text	String	Yes	The markdown text to render
gfm	Boolean	No (optional)	Render text using GitLab Flavored Markdown
project	String	No (optional)	The full path of a project to use as the context when creating references using GitLab Flavored Markdown

Notes

1. There is no option to chose a markdown engine. The common_mark is used because [we are migrating to CommonMark] (#43011 (closed)).
2. The endpoint was added only to APIv4 because APIv3 is deprecated and about to be dropped.

Todo

• add documentation
• add specs
• leave a comment in the issue when this MR is merged

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

No.

Why was this MR needed?

Improve API

Screenshots (if relevant)

No.

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
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #19861 (closed)