Proposal: Switch local docs theme to Readthedocs

[pgagne]

When building docs locally it would be nice to use the same theme as readthedocs so the output looks the same. This seems pretty easy to do:

https://github.com/rtfd/sphinx_rtd_theme

Notes

We should add sphinx-rtd-theme to docs/requirements.txt.

If people are unsure about the added dependency, we should be able to add handling to sphinx conf.py to not set html_theme if sphinx-rtd-theme is not installed. This would result in the default theme being used.