WIP: Add preview for new markup files

What does this MR do?

This MR implements the preview markdown feature for new file described in #20303 (closed)

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

NR

Why was this MR needed?

This improve user's editing experience

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #20303 (closed)