freebsd-base.7: New manpage describing pkgbase

This covers the basic structure of pkgbase, how packages are
distributed, how package sets work, and provides a few example
of actually using pkgbase.

MFC after: 6 hours
Approved by: re (cperciva)
Reviewed by: ziaee, cperciva
Differential Revision: https://reviews.freebsd.org/D52644