Rework the Sphinx find module

* Make sphinx_rtd_theme a find module component to avoid hard-wiring it
  into the module and allowing to report the theme in case it is missing
  using the standard CMake package mechanism.
* Adjust find module cache variables names case to match the find module
  name.
* Also report sphinx-build version for completeness.
* Invoke the find module only once. Calling find_package on the same
  module is not needed.

Change-Id: I9d1bf0fcc0d44b9b37e624128812f348c5442ada