Documentation for pid4cat service (not just for the model)

[dalito]

The current mkdocs based documentation should be extended to cover not only the linkml metadata model but all aspects of pid4cat (model, APIs, python package, best practice guides etc.).

[dalito]

#97 significantly extends the documentation to cover most aspects of the pid4cat service. However, some parts are still in early draft state.

Before closing this issue the results of a currently ongoing documentation review (in google docs) should be evaluated and used to prepare a second PR with improvements.

The extended docs based on #97 are already deployed as "dev" version. See here https://nfdi4cat.github.io/pid4cat-model/dev/