Added Gerrit to git integrations,and updated structure and content of git triggers

Added list of trigger events per provider supported by Codefresh

Replaced description of Host URL for Gerrit; added mapping for Ref updated trigger

…docs.codefresh.io into gerrit-integration

Removed Push heads event from list of Gerrit triggers

Added Gerrit requirements to git integration, git triggers, and variables

Implemented  review comments for Classic

Added draft with initial screenshots

Added gerrit credentials to hosted runtime and edited Gerrit integration

Updated permissions and HTTP password reqs in Gerrit; added section on using gerrit in codefresh

Added link to SSH keys for Gerrit to official documentation