Provide git-extras as tool for CLI integration instead of custom documentation

When in a MR there is a message hint for checking out a MR locally as seen here:

The "tip" points to documentation in https://gitlab.gnome.org/help/user/project/merge_requests/index.md#checkout-merge-requests-locally.

Those instructions are quite cryptic, seems this would be more useful if instead mentions to install git-extras. Linux, Windows and Mac are supported installations and it provides several helpers that work across systems and come quite handy for users used to CLI.

In case this is wanted, I could do the change.