use MDA theme for docs

[orbeckst]

While we're at fixing docs (#65 #66) we should also update to the MDA sphinx theme.

[orbeckst]

@lilyminium is there a basic "howto" for updating to the MDA sphinx theme?

[IAlibay]

@orbeckst I have a PR open for this - see #64

[orbeckst]

Ah, nice, hadn't seen it.