Add page for Gerrit to integrations

Created by: christinaforney

@ryphil and @Joelkw - can you help fill in the contents of this page in the areas each of your teams own? Don't need anything super robust here, but this is currently missing and we should clarify what is supported and how to configure. (This should just be a quick task to fill in details folks on your teams know, not a big effort to fully flesh out documentation).