Revert "docs: use html_static_path for static files"

No, html_static_path doesn't do the same thing as html_extra_path; it
puts things inside the _static folder, which we don't want here. Let's
revert the change.

This reverts commit e037761a2f.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29805>

docs/conf.py

@@ -111,8 +111,10 @@ 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 = [
-  '_static/',
+html_static_path = []
+
+html_extra_path = [
+  '_extra/',
   'release-maintainers-keys.asc',
   'features.txt',
   'libGL.txt',
@@ -120,8 +122,6 @@ html_static_path = [
   'README.VCE',
 ]
 
-html_extra_path = []
-
 html_redirects = [
   ('webmaster', 'https://www.mesa3d.org/website/'),
   ('developers', 'https://www.mesa3d.org/developers/'),