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>

docs/helpwanted.html

@@ -47,7 +47,7 @@ You can find some further To-do lists here:
 <b>Common To-Do lists:</b>
 </p>
 <ul>
-  <li><a href="https://cgit.freedesktop.org/mesa/mesa/tree/docs/features.txt">
+  <li><a href="https://gitlab.freedesktop.org/mesa/mesa/blob/master/docs/features.txt">
     <b>features.txt</b></a> - Status of OpenGL 3.x / 4.x features in Mesa.</li>
 </ul>