Revamp Sphinx theme

[jezdez]

We use the custom conda Sphinx theme in a number of repos which makes it much harder to update the way the docs look like.

As a first step we should move the Sphinx theme in a separate repo, e.g. conda-sphinx-theme, and enable it in all repos that make up conda.io. This work is in preperation of #747 and other related issues.

[jezdez]

/cc @jaimergp