Use Plone Sphinx Theme for brand, consistency, and features

[stevepiercy]

The Alabaster theme is 💩. It is worse than Basic theme.

I'd like to use Plone Sphinx Theme on it, now that it is ready for general use. A consistent brand and identity is a Good Thing™.

Then we can link to it from wherever bobtemplate.plone is mentioned in the primary docs.

Originally posted by @stevepiercy in #570