doc: use ReadTheDocs.org theme if available

Instead of including the rtd theme in zephyr use an installed instance,
if nothing is installed, use the default zephyr theme.

Change-Id: Ife4bd878e3f879cf59ecf2bc5d186a531a3bf1b6
Signed-off-by: Anas Nashif <anas.nashif@intel.com>