Move non-reference docs for couchbase-cli and cbbackupmgr to a private docs repo

As discussed on 4/10, we'll move docs for couchbase-cli and cbbackupmgr to a new private repo managed by docs.

Note that we'll not move reference docs–these will remain in engineering-managed repos.

The scope of this effort includes doc for 8.0 (Morpheus). We will not move docs for previous versions.

cc: Hakim Cassimally , Richard Smedley , Matt Hall , Daniel Owen , Nour Elmenyawi