Fix overlap of right-sidebar and wiki create page form

What does this MR do?

• Fix overlap of right-sidebar and wiki create page form

Before	After

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

• We don't seem to have any existing tests for sidebar classes on specific pages. Do we want to introduce something around this?

Why was this MR needed?

Right-sidebar was overlapping the main create wiki page form content. This happens when there is an error because we see the POST /<namspace>/<project>/wikis page rendered out.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #39509 (closed)