JupyterLab extension to browse jupyter books from the interface

[choldgraf]

It would be really cool if we had some lightweight JupyterLab interface elements that would let people step through notebooks the same way that they do a Jupyter Book. I can think of two things to add:

1. Add a "previous" and "next" button for each page using the _toc.yml file. Similar to these buttons:

   

2. Add a toc.yml browser to the left sidebar that behaves just like the file browser, but where the files are ordered according to toc.yml, not the naming of the notebooks.