Documentation changes.

• Explicitly request the read-the-docs style. Running locally has always required us to request sphinx_rtd_theme. Previously, readthedocs.org would use its eponymous style (sphinx_rtd_theme) by default. This appears to have changed, so request sphinx_rtd_theme no matter where we are running.
• Remove references to the breathe python module since it is unused.
• Remove some useless whitepace.