Add documentation

[pbrehmer]

This PR will document all relevant functions and types, and provide a proper README.

[pbrehmer]

I think most parts of the code should be documented now (up to less relevant, internal functions). What is left to be done is add a proper README and maybe document the examples with some explanations.

[lkdvos]

Do you want to wait with the README or go ahead and do it right away? I am very much okay with already adding these docstrings, it's not that much work to change them afterwards if need be.
I also still want to make some minor changes and clean up some of the files/folders, so it's better to first merge this such that we don't get merge conflicts, as writing docstrings touches large parts of the codebase.
Will you mark it as non-draft whenever you feel like its ready? I'll try and review asap then (allthough at first glance looks great)

[pbrehmer]

I think we can add the README also later. But I will add some comments to the examples tomorrow morning. Then this PR should be ready to merge! I hope this will avoid most merge conflicts.

[pbrehmer]

This should be ready to review now. I haven't yet added any comments for the test scripts in PEPSKit.jl/examples/ since they will be converted to proper tests anyway at some point.

[lkdvos]

Looks good to go for me!