man: describe naming schemes in a new man page

I decided to make this a separate man page because it is freakin' long.
This content could equally well go in systemd-udevd.service(8), systemd.link(5),
or a new man page for the net_id builtin.

v2:
- rename to systemd.net-naming-scheme
- add udevadm test-builtin net_id example