Add an example of an exercise with a hidden solution

[danielballan]

This uses cell metadata to make a cell hidden by default but expandable, both in JupyterLab and in the rendered HTML.

---
tags: [hide-cell]
jupyter:
  source_hidden: true
---

The tags operate on MyST. The jupyter key operates on JupyterLab. See references below.

In JupyterLab

References:

• https://jupyterlab.readthedocs.io/en/3.5.x/api/interfaces/nbformat.icodecelljupytermetadata.html#source_hidden
• https://discourse.jupyter.org/t/hiding-code-cell-on-launch/1763/10?u=danielballan

Rendered in HTML by MyST

Reference: https://jupyterbook.org/en/stable/interactive/hiding.html#hide-entire-code-cells

[bsipocz]

🎉