Bonus exension and jupyter book hints

These commits are me fixing a bunch of issues in the course website, which is a built with jupyterbook.

3bb9bfa fix typo and extend early bird
8dd1b05 fix metadata on cell syntax
ec7423c add file extension on include
d1976e6 unnest code block
68adcdf fix typo in kbd directive
4b2bc4e questions
9131d0e link toc