v23: Move v.io/v23/mgmt to v.io/x/ref/lib/mgmt
v23: Move v.io/v23/mgmt to v.io/x/ref/lib/mgmt

Also add package doc.

MultiPart: 1/2
Change-Id: Idf4279de01caefeb1543c404ef17475dda1ffae3

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23