git-for-each-repo: add page

[navarroaxel]

• 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

[bl-ue]

Left some comments. Also, if possible, could you please add an example for running an arbitrary command (i.e. not a git subcommand)

[navarroaxel]

Left some comments. Also, if possible, could you please add an example for running an arbitrary command (i.e. not a git subcommand)

The man page says this is only for git commands.

DESCRIPTION
Run a Git command on a list of repositories.

But maybe we can add a shell command as a git subcommand alias and that could work. 🤔

[bl-ue]

Oh, I didn't notice that part :). No, it's fine how it is, I think.