misc doc improvements and theme update

[OriolAbril]

This is first to test everything works. It should not be merged.

Once everything is tested across sites I will make a release of the theme
and that should be the dependency

[OriolAbril]

I just noticed that now with the global search working the bart related notebook appear twice:

Where do we prefer users read them from, the pymc-examples gallery or the bart docs? We should probably add them to the ignore section of the .readthedocs in the non-preferred place. They will still appear in the local search for both websites but only once for the global one

Note: the search was done from within pymc-examples so the pymc-bart gets the "from project ..." whereas the other doesn't

[aloctavodia]

LGTM. Global search should show the ones in pymc-examples.

[OriolAbril]

Search settings updated, I think there is nothing left to do