Move auto-generated docs to main stage
Move auto-generated docs to main stage

Move the docs to the main stage and make a phony rule to build
them.  This will allow the primaray builder to choose not to
build them so that it can use its own rule instead.

Change-Id: Ib788f74717ccdcd85c2e5b6a581cc744d439341a

google/blueprint,danw/blueprint,google/blueprint,danw/blueprint