Improve documentation of basic course scenario

Problem: The dumux course has to be set up in a specific way to work as expected. Well documentation will help beginners.

During my work on doing the dumux course (basics exercise), I found that the documentation for beginners could be improved.
This Issue is to discuss this and avoid spam in the mailing list for everyone who is not involved. Thanks @timok for your response 👍.

Problem and solution description

My intuition was that it would be possible to

clone the dumux course repository into the existing dumux installation and then
build the project files in the moment when they are needed.

This may be possible, but leads to confusion because the steps in this case are different from those in the Tutorial description.

My solution that works was to

copy the installexternal.py-file from myInstallationFolder/dumux/dumux/bin to myInstallationFolder/dumux,
change to the new directory of this file (cd myInstallationFolder).
run python3 installexternal.py course to add the dumux-course to the existing DuMuX-installation and follow the steps mentioned while the installation is running.

After the installation is complete I could remove the installexternal.py file from this directory to clean up.

The way @timok proposes is as follows:

clone the dumux course repository into the existing dumux installation,
configure using CMake or dunecontrol and
build the Dune modules and Dumux libraries you want to use.

Did I got that correct?

How to improve this

In order to improve the documentation, I see two possibilities:

Extend the Preparing the exercise section in the basics exercise with my solution or the one proposed by @timok or to
Give more details on how to get the course on the getting started guide on the homepage.

I'd like to implement this, please let me know which way the community prefers. I think both solutions are possible without duplicate resources, which avoids maintenance effort.