documentation: Clean up references to 'Gerrit2'

Now that the 2.x code base is over a year old, and has been in
production for almost a full year, its time to stop calling this
'Gerrit2' and start referring to it by the complete project name,
Gerrit Code Review.

Change-Id: Ifb37dd63b94ed7e1b938511aa0fc2a6ad3c981d6
Signed-off-by: Shawn O. Pearce <sop@google.com>
19 files changed