Fix relative links in Markdown wiki when displayed in "Project" tab

What does this MR do?

In !6396 (merged) we introduced a possibility to disable a repository for a project. In this case we display wiki homepage in "Project" tab. This MR fixes relative links in Markdown wiki homepage when displayed in "Project" tab.

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
• [ ] 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?

Refers to #23806 (closed)