Consider adding RTD badge to sidebar

[bitprophet]

Pros: badges are cool, supporting RTD is cool.

Cons: Given most Alabaster sites are likely to be hosted on RTD it feels a little silly having a link-to-self in the sidebar; and if the build is broken, how will you know? You're either seeing some working version of the docs, or you're seeing a 404 page (and not the badge) XD