Further enhance the read the docs documentation

[HenrZu]

Provide a description of requested documentation changes.

As merged in #1224, we introduced a basic read the docs documenation. We want to enhance this even more.

[kilianvolmer]

#67 (comment) already lists some points that should be answered by the How-To advices.

[kilianvolmer]

Comment by @xsaschako to #1224 :

two things:

    delete simulation folder, when every one is transitioned to other github -> where do we link that?
    Readmes? E.g. main readme still has link to old docu

[mknaranja]

In this issue, we should also update the lower part of the readme on the github entry page.

[jubicker]

Please delete the docs/models folder (old ReadMes) before merge.