Switch to MyST-NB (instead of nbsphinx) for rendering doc examples

[alexenge]

This means that:

• We can render the examples from .qmd files (using jupytext and Quarto) – Note the for this to work, atm we need to set some Quarto-specific environment variables in conf.py because the Quarto installation (via conda-forge) on the Read the Docs server is a bit f***** up (all works fine locally)
• The input/out cells look a bit nicer
• There's way more configuration options in MyST-NB, e.g., for hiding code cell inputs or outputs
• Unfortunately, no more Sphinx Gallery for the examples (but see https://github.com/executablebooks/MyST-NB/issues/396)

Fixes #131