docs: include specs into the generated docs

Unfortunately, it doesn't seem like there's a way to have sphinx copy
this without moving the files, becasue html_extra_path doesn't copy the
directory itself when given a directory, only files inside and
subdirectories.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4630>

docs/conf.py

@@ -113,6 +113,7 @@ html_copy_source = False
 html_static_path = []
 
 html_extra_path = [
+  '_extra/',
   'features.txt',
   'libGL.txt',
   'README.UVD',