handbook/ports: file:// URLs for poudriere repos

FreeBSD Handbook: …: Configuring pkg Clients to Use a Poudriere
Repository

Add a small section showing the user how to point to their packages
repository (package directory in this case) via file://. Allow the user
to not publicly expose their packages over the Internet.

https://bugs.freebsd.org/263955

PR: 263955
Reported by: Jonathan Vasquez
Reviewed by: debdrup, pauamma
Pull request: https://github.com/freebsd/freebsd-doc/pull/66