Improve git-signing page, make it work with all OS'es

[sofusalbertsen]

I read through it and it looks quite ok, especially for a first edition.
It will probably need some additions later to not be so Windows specific (i.e. which gpg tool to use on linux/mac etc.)

Also, I believe that on newer versions of gpg --full-generate-key it no longer has default values, so you have to explicitly choose key type and key length.

Let us keep an eye on these gotcha's when they show up in training, and improve the guide later.