docs: specify redirects in conf.py instead

This makes it much easier to discover these.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7174>
diff --git a/docs/conf.py b/docs/conf.py
index a03fc16..1d16c5a 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -135,6 +135,12 @@
   'README.WIN32',
 ]
 
+html_redirects = [
+  ('llvmpipe', 'gallium/drivers/llvmpipe.html'),
+  ('postprocess', 'gallium/postprocess.html'),
+  ('webmaster', 'https://www.mesa3d.org/website/'),
+]
+
 
 # -- Options for HTMLHelp output ------------------------------------------