man: add udev.conf(5)

We generally have separate man pages for all configuration files.
In this case udev.conf was already described in systemd-udevd.service(8),
but it was hard to find. Docbook makes it hard to add a .so link from
a different section, so describe udev.conf in its own page.

Signedaoff-by: Anthony G. Basile <blueness@gentoo.org>