R: rename to r

[bl-ue]

• The page (if new), does not already exist in the repo.
• The page is in the correct platform directory (common/, linux/, etc.)
• The page has 8 or fewer examples.
• The PR title conforms to the recommended templates.
• The page follows the content guidelines.
• The page description includes a link to documentation or a homepage (if applicable).

For #5377

[owenvoke]

Going to leave this unmerged, just to double check that it's ok.

[bl-ue]

@owenvoke I think it's probably fine. Notice pages.fr/common/r.md and pages.pl/common/r.md are both lowercase, and they are both the same R.