Added support for custom wiki sidebar using _sidebar.md in the wiki root. If…

What does this MR do?

Added support for custom wiki sidebar using _sidebar.md in the wiki root. If _sidebar.md exists it will be parsed and rendered otherwise the default sidebar file list will display.

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

Why was this MR needed?

There was no way to have a custom sidebar navigation for a wiki and the default option didn't work well for very large wiki. See issue 14995

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
• Internationalization required/considered

What are the relevant issue numbers?