Only included commit about config

rebased 2 times master and refactored commits, improving typing

Separated commits, improved config loading

Changed to install pre-commit as a python package in venv.
Removed PATH adjusting instructions since they are no longer needed here.

In D3867#95583, @vlorentz wrote:

if the venv needs to be configured before pip, then pip install pre-commit should be below

The idea is to install it outside of a venv, to access it like the system package.
But I chose to install it as user (no pip install --system) because I think it shouldn't need to reside in root territory.
We can discuss the preferred approach :

• system or user?
• necessarily py3?

I don't think it needs to be py3 because instructions mentioned the Debian package which is py2 I think.