Add link to Third-Party CI group in Gerrit docs

This is a valuable link for folks looking to contact people running the
3rd party testing that comments on changes, but it's not simple to find
even if you know it exists. Adding link in docs so it's easier.

Change-Id: Ia19417a9105db6509489cc5aac4e18a4ee216856
Merge "Add link to Third-Party CI group in Gerrit docs"