Migrate Global JJB docs to RST

Migrate Global JJB docs to RST

This moves Global JJB documentation into RST docs and adds a
new checker which parses YAML files for items that are missing
documentation to ensure that every new template addition comes with
supporting documentation.

Tried to update documentation where it made sense too but we should
probably go back later and review all the items to make sure they are
all sufficiently documented. Especially macros which in many places are
lacking in required parameter documentation.

Change-Id: I6551e14f934a904378771262d1491cb5b3df5854
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>