docs: rework stuff

The existing docs were far from how we wanted it to be. There was so much to do
and it is not feasible to do that cleanup it in clean patches.

Instead, I took a sweep through the docs and changed what I thought could
benefit from a change: structure, examples, advices, language, markup, content,
etc.

First step in two-part process to rename directories to kallithea.
This first step is to change all references in the files where they refer
to the old directory name.

fix for issue #417, git execution was broken on windows for certain commands.
Added git as requirement into readme.