DOC: Some more vis cleanups
Two things included:
1. Try to use ggplot style. If not, fall back to the previous 'default'.
2. Fixes some warnings in the doc build (part of https://github.com/pydata/pandas/issues/8234). There are some remaining warnings that I didn't have a chance to track down. Will see if I can later.

@TomAugspurger I am going to merge this, as I want to build the docs for the release candidate. The remaining warnings can then be fixed before final release.
