docs: do not copy source-files to site

These docs have publically available sources in the first place, there's
no point in including a copy of them here as well.

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

docs/conf.py

@@ -103,6 +103,8 @@ html_theme_options = {
   'display_version': False,
 }
 
+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".