Split existing tutorials up into documentation on different levels

[apirogov]

Currently, the Notebooks are in the uncanny valley between various kinds of documentation, while being not fully satisfactory on any single "level".

They should be broken apart and cross-linked.

There should be:

• concepts (high abstraction level)
• tutorial, bite-sized examples (concrete, copy-paste-able)
• mid-level manual (explanation beyond what is in the docstrings)
• automatic docs (the technical reference, generated from docstrings)
• pure user docs (for users not writing code)