Remove dependency to 'n42org/tox:3.4.0'

### Current Status

`n42org/tox:3.4.0` is used to build the documentation.

Problems:

* setup does not work with `docker in docker`
* #4132
* this software is unmaintained (see https://github.com/n42org/docker-tox)
* not very clear who the publisher is
* (https://github.com/n42org)
* https://registry.hub.docker.com/u/n42org
* https://n42.org/

### Expected

tox is called more directly, without the need for n42org.

`docker in docker` should work (e.g. when creating multiple containers with different setups, edited via e.g. VSCode Container mode)

### Solution

Execute tox directly, should be just a few cli steps within the makefile

### Please let us know if you plan to work on this.

Yes!