Rework Ceres documentation as new website

This reworks the Ceres Sphinx documentation such that it can
function as the main Ceres website, now hosted at
ceres-solver.org. This also changes to the theme sphinx_rtd_theme
used by Read The Docs; this theme has strong mobile support and is
well enough designed.

Change-Id: I63232d985859a6dac94ff58f08bf81eb2b9e7f99