Add meta descriptions to pages

[rsokl]

We can improve PLYMI's Search Engine Optimization score by adding meta descriptions to our pages.

This can be achieved via rst-directive in sphinx. Via nbsphinx, this requires us to use a raw-rst cell. However, this is blocked by current limitations in jupytext-markdown. Hopefully this will be remedied soon.

[rsokl]

#85 closes this 🙌