DOC: Overview of Nixpkgs/channels/Hydra/tests (close #11263)

What I missed when I began using Nix and NixOS was a clear overview of
how packages, channels, Hydra, the master branch and updates to channels
relate to each other.

I've noticed I am not the only one, given the amount of times these
questions pop up.

For now I propose to include this in the Nixpkgs manual, since this
seems to be the best fit. However, I think it would be good to include
this in either a new manual, i.e., a user manual, or an 'official'
tutorial.