docs: bundle extra files

These are documents that are bundled in the root of the website, and
contains some useful, extra documentation. Let's include them.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4630>
diff --git a/docs/conf.py b/docs/conf.py
index 26beed3..d5e0521 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -112,6 +112,15 @@
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = []
 
+html_extra_path = [
+  'features.txt',
+  'libGL.txt',
+  'README.UVD',
+  'README.VCE',
+  'README.WIN32',
+  'VERSIONS'
+]
+
 
 # -- Options for HTMLHelp output ------------------------------------------