Use relative URLs when linking to uploaded files

What does this MR do?

Switches from absolute to relative URLs when linking to files uploaded to a GitLab instance

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

Why was this MR needed?

Without this, the cached markdown can be invalid in a range of contexts, from secondary Geo nodes to renamed instances.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• 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?

Closes #38893 (closed)