Enhance CRD doc with diagrams

Rely on blockdiag to illustrate the CRD examples.

Change some references to "check" and "gate" to respectively
"Independent pipeline" and "Dependent pipeline". Since third parties
might use a different terminology.

Decorate note with '.. note::'

To render blockdiag nodes as circles, we need blockdiag 1.1.0. Bump the
test requirements for sphinxcontrib-blockdiag to at least 1.1.0.

Change-Id: I4b0600309bbc3bc238c80c5ea78d9d3c233b6ef2
2 files changed