Doc: enhance gating reference with diagrams

ASCII art is fun but not savvy for project managers and directors. This
patch slightly enhance the 'gating' documentation with colored diagrams.

This is made possible via http://blockdiag.com/ by Takeshi KOMIYA who
even took the time to write a sphinx extension.  The version dependency
is at the very least 0.5.5, but might be higher :/

Change-Id: Ibe3c2674a5dff2114c40a84ffdec8a8886b1b21b
Merge "Doc: enhance gating reference with diagrams"