Update docs about how to hide code cells

[davidwagner]

The FAQ part of the Jupyter Book guide says the way to hide code cells is to set hide_cell_text in _config.yml. See https://jupyter.org/jupyter-book/guide/05_advanced.html. I think this is out of date and no longer works, and now one must set metadata tags in those cells. I suggest updating this part of the docs.

[choldgraf]

@davidwagner good catch - we should definitely fix this before the next release