Complete rewrite of documentation, moving from Sphinx to mkdocs

[lonvia]

Drops Sphinx in favour of mkdocs for generating the documentation. Needs mkdocs, mkdocs-material, mkdocstrings and mkdocstrings-python packages for generation.

The documentation itself also has seen a major overhaul:

• The user manual is heavily extended and mostly focusing on the new iterative way of reading data.
• There are a couple of cookbooks which are written in the form of Jupyter notebooks.
• The code documentation for the C++ bindings has moved to the .pyi type files, which makes it easier to read and edit. mkdocstrings is also much better in generating docs from the Python type files than from the docstrings generated by pybind11.