Improve Documentation

[mrossinek]

The documentation is currently limited to some minimal API docs and a single end-to-end workflow tutorial (which is not tested during CI).
This should be improved by the following:

• code examples in the API docs (see also the comment regarding doctests in Improve Test Suite #7)
• more fine-grained and targets tutorials and how-to guides (which also get tested in CI, see also Improve Test Suite #7)

We should also guide users on how to deal with certain scenarios like the following:

• working with a target observable consisting of more than a single Pauli term