networking.7 : create network quickstart guide

Now that the handbook has been moved to ports, I think it's very nice to
have a network quickstart guide in-band, in base, in the system manual.
If the user uses any of the following terms "man -k
{network,networking,wifi,quickstart}" this page will come up, which is I
think a very common use case for new users.

Currently, this document explains connecting to a basic Ethernet
network, a basic wifi network, scanning for wifi networks, and airplane
mode, as well as linking to other sections, including the handbook

Co-authored-by: Graham Perrin <grahamperrin@gmail.com>
Reviewed by: imp, bcr, freebsd@igalic.co
Pull Request: https://github.com/freebsd/freebsd-src/pull/833

(cherry picked from commit 75eda0096bf83b9b88996d9903b5d4e504047eba)
(cherry picked from commit 5dced41439109368a8f3c9a818fe518542d7063e)

Approved-by: re (cperciva)