docs: cat maintainer keys to a single file

The original issue asked for all the keys in a single file, but I didn't
do that because it's much easier to manage and verify the keys as
separate files, but sphinx doesn't provide a way to expose a folder so
we'd need to create an index.html and have it list all the keys
manually, which is very error prone.

At this point, we might as well just concatenate the keys and expose
a single file, so let's do that.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5568>

docs/conf.py

@@ -114,7 +114,7 @@ html_static_path = []
 
 html_extra_path = [
   '_extra/',
-  'release-maintainers-keys/',
+  'release-maintainers-keys.asc',
   'features.txt',
   'libGL.txt',
   'README.UVD',