docs: use html_static_path for static files

I'm not sure why I picked html_extra_path instead, as it's meant for
slightly less directly related files than what html_static_path is for.

So let's switch. It shouldn't make much of a real-world difference, but
should make it a bit easier to understand what this is about.

Reviewed-by: Corentin Noël <corentin.noel@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25585>

docs/conf.py

@@ -109,10 +109,8 @@ html_copy_source = False
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path = []
+html_static_path = [
-
+  '_static/',
-html_extra_path = [
-  '_extra/',
   'release-maintainers-keys.asc',
   'features.txt',
   'libGL.txt',
@@ -120,6 +118,8 @@ html_extra_path = [
   'README.VCE',
 ]
 
+html_extra_path = []
+
 html_redirects = [
   ('webmaster', 'https://www.mesa3d.org/website/'),
   ('developers', 'https://www.mesa3d.org/developers/'),