docs: update repo URLs after GitLab move

docs: update repo URLs after GitLab move

I also updated the developer instructions; presumably someone who's been
given commit rights already knows how to clone a repository :)

A more useful thing is to show how to update the pushurl, and how to use
access tokens to push over HTTPS (especially for us at Intel, where
non-http traffic is a pain).

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>