Too narrow project READMEs on fixed layout

What does this MR do?

Makes project README containers wider on fixed layout.

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

n/a

Why was this MR needed?

The width of project READMEs is too narrow compared to that of its parent container on fixed layout. This bug would be introduced in !12165 (merged).

Screenshots (if relevant)

before	after

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• [n/a] Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [n/a] Has been reviewed by Backend
  • [n/a] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [n/a] Internationalization required/considered

What are the relevant issue numbers?

Closes #41600 (closed)