Demonstrate markdown outputs and plotly support

[choldgraf]

Per a question from @rowanc1 - here are two features missing that I'd need before deploying mystmd at 2i2c.org/kpis.

1. Plotly support. Plotly requires its own JS bundle and can be hard to get properly. JupyterBook recommends that you manually link the require.js package but I don't believe this is possible in mystmd yet. This is possible after all! I debugged it and learned that I needed to explicitly call fig.show. I've updated the docs now :-)
2. Markdown outputs parsed as myst. Currently the markdown outputs aren't parsed as anything (though weirdly, they are in JupyterLab MyST).

Both of these can be demoed in this PR at the docs preview link below :-)

Deploy preview link

[agoose77]

Requires fixing #1026

[choldgraf]

Closing this PR because it's not actionable. Here's the issue that will need to be resolved in the future:

• Parse markdown outputs in notebooks along with the rest of the markdown on a page in order to programmatically generate MyST content #1026