Docs fixes

[QuantumChemist]

some corrections, adjustments and fixes of the documentation

• add more documentation on the choice of the fitting parameter for the phonon workflow. maybe, provide the Si choice as a default or explain how to chose it
• add documentation on how to allow users to adapt VASP settings more easily

[QuantumChemist]

@JaGeo , here is the new PR.

If the issue with the stuck test is fixed and you are happy with the changes, we can merge this PR.

[JaGeo]

I will merge once docs and linting are through. Then, we need to work on the tests

[QuantumChemist]

I will merge once docs and linting are through. Then, we need to work on the tests

Ok, now everything except the stuck test has passed. And in principle test split 5 passes as well as one could see in recent CI runs.

[JaGeo]

There are some conflicts now. Could you resolve them, @QuantumChemist ?

[QuantumChemist]

There are some conflicts now. Could you resolve them, @QuantumChemist ?

solved! :)