git-cat-file: add page

[qhyun2]

• 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).

[tldr-bot]

The build for this PR failed with the following error(s):

pages/common/git-cat-file.md:3: TLDR004 Command descriptions should end in a period

Please fix the error(s) and push again.

[qhyun2]

#3953

[sbrl]

Thanks, @qhyun2!