Fix: Update Sphinx to fix RTD build

The old version pinned in requirements.txt was causing build failures
on RTD. This has been updated, along with a few small fixes that
showed up once the build was working again.

Issue: RELENG-5138
Change-Id: Icaa18d3a39540148b328654bddb9d7fb98ecbd31
Signed-off-by: Eric Ball <eball@linuxfoundation.org>