Consolidate UI page widths - Part 06: Merge requests

What does this MR do?

Consolidates page widths according to https://gitlab.com/gitlab-org/design.gitlab.com/issues/47, focused on Wiki pages.

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What are the relevant issue numbers?

• Design pattern: https://gitlab.com/gitlab-org/design.gitlab.com/issues/47
• Implementation issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/51439

Issues

Closes #51439 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides