Add gates for API projects and operations-guide

This change uses the same setup that is used for api-site already:
* Create checks and gates for niceness, syntax, deletions, build
* Make checkniceness non-voting

It enables these for all API projects and the operations-guide.

Removes also the old operations-guide gate job since it's not used
anymore.

Change-Id: I1cc9f50e40ff422e7bbcf2cb581292f2c79c3f71
Merge "Add gates for API projects and operations-guide"