Update docs theme and packages

[briantist]

There's some rendering weirdness in our current docs, like table of contents showing up as a series of lists that aren't formatted very well.

See for example:

• rendering wrong in docutils 0.17 (unordered list, maybe more?) readthedocs/sphinx_rtd_theme#1115

We're long overdue to update these things. Related (should probably be done at the same time):

• Move docs dependencies into their own group #1053

[briantist]

Resolved by #1119