git-checkout-index: add page

[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 #3953

[sbrl]

Thanks for the new page! I've left some comments below for you to review.

already done