Initial docs for Jenkins integration

Initial docs for Jenkins integration

Add basic documentation for (some) of the Jenkins builds from the
perspective of how they interact with the build system.  This part
cannot change without changes in the build system and so can be
documented in the same repository as the build system.  And the
developer guide is a place where people would (hopefully) first look for
this kind of information.  Links can be provided to more documentation
on Jenkins usage and other configuration (e.g., on the wiki), once that
stabilizes.

Related to #1731.

Change-Id: I87e180d12a77c692ac4594b6c062fb6ca150aa5d