docs: show 'Edit on GitLab'-link

It's easier to contribute to the documentation if we have links to the
document on GitLab. This will allow people to easily edit docs, or to
realize where in the source-tree they are without having to search.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6742>

docs/conf.py

@@ -109,6 +109,15 @@ html_theme_options = {
   'display_version': False,
 }
 
+html_context = {
+  'display_gitlab': True,
+  'gitlab_host': 'gitlab.freedesktop.org',
+  'gitlab_user': 'mesa',
+  'gitlab_repo': 'mesa',
+  'gitlab_version': 'master',
+  'conf_py_path': '/docs/',
+}
+
 html_copy_source = False
 
 # Add any paths that contain custom static files (such as style sheets) here,