Revamp documentation

[nanne-aben]

Big revamp of the documentation. Content is largely the same, but reorganised things. Amongst other thing:

• Fewer things are hidden under advanced
• Notebook functionality is more pronounced
• StructType has its own page (instead of being spread over three pages)

You can see the documentation of this branch here: https://typedspark.readthedocs.io/en/revamp-documentation/