Move documentation to use jupyter-book instead of nb2plots

[jni]

nb2plots appears to be unmaintained and appears to have stopped working with recent versions of nbconvert (6.0). This was confirmed by @GenevieveBuckley who couldn't build the .rsts until she downgraded to nbconvert 5.6.1.

Jupyter Book is an excellent alternative for docs and is well maintained. See https://github.com/napari/napari and https://github.com/napari/tutorials for jupyterbook configuration examples.