Add playground to docs

[waylan]

Zensical provides all of the necessary assets for Pyodide, Ace, and Highlight.js out of the box as part of its support for the Markdown Exec plugin. We just need to provide the HTML. I have manually crafted the HTML that Markdown Exec would render.

Fixes #1628.

Description

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Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
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[waylan]

My thinking here is that Python-Markdown is a library first and foremost. Therefore, we should provide a Python environment for playing with the library. As it turns out, Zensical makes it extremely easy to provide that. We could always add an alternative playground which works more like the demo at https://dingus.python-markdown.workers.dev/ in the future.

I considered creating another custom fence for this, but as it is not going to be reused, the single page of raw HTML is simpler to implement and update.

[waylan]

[facelessuser]

Yep, I think this is a reasonable solution. I was working to reduce some of my related work to something far simpler, but I honestly forgot that Markdown Exec existed. So yeah, I think this is a good solution for what we want here.