Make blob viewer wider for mobile

What does this MR do?

Makes the blob viewer for rich contents (e.g. .md) wider for mobile.

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

While this MR contains media query keeping the relevant declaration, is this truly needed? If not, we can remove the declaration.

Why was this MR needed?

On a mobile device, the blob viewer for rich contents (e.g. .md) is too narrow to read. This MR improves UX for that.

Top/Bottom margins are unchanged as is before !12198 (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
  • [n/a] 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

What are the relevant issue numbers?

Closes #37368 (closed)