Adds documenation based on tests to AthenaMonitoringKernel
As discussed via e-mail among: @ponyisi @fwinkl @dzanzi @morgens I have added documentation of how to use Moniored classes. The examples are based on actual unit tests and therefore are guaranteed to be correct (as long as the tests are succeeding).
They are added for Monitored::Scalar / Collection and Timer.
Example: https://tbold.web.cern.ch/tbold/docs/html/classMonitored_1_1Scalar.html#details
It is possible thanks to the fix from @fwinkl !ATLINFR-3391