docs: Renaming /go/ top-level directory to /topic/

Per discussion with JF and DD, the new holding pen for not-quite-
top-level topics will be a "/topic/" directory, not "/go/". (Currently
the only thing living there is the new "libraries" section.)

Updating redirects and TOCs accordingly. See first comment for
doc stage location.

bug: 28558659

Change-Id: I09201a8334cac70d76559a106a74d5a29fe82128