Try out sphinx-exercise for exercise formatting

[scottyhq]

https://ebp-sphinx-exercise.readthedocs.io/en/latest/ This allows nice formatting and referencing of exercises on the rendered website. I tested things out with https://tutorial.xarray.dev/fundamentals/02.3_aligning_data_objects.html#alignment-putting-data-on-the-same-grid

Unfortunately the more special myst and sphinx markup in .ipynb files can lead to somewhat odd appearance in JupyterLab. But I think requiring a bit of effort to reveal the solution is good to force thinking it through. Some screenshots below on how this looks unrendered and rendered:

Rendered: Current (left) and This PR (right):

Code: What a user sees in JupyterLab (left), actual markdown (right)

If solutions don't require code execution (variables from earlier in a given notebook), they could be rendered on an entirely separate page (reference/exercise_solutions.md)...

A more lightweight solution is just to have separate mardown cells with headers (###solution) and use the 'hide-output' metadata tag to reveal on the website