docs fiddle: Pre-defined outputs / caching

[Akeboshiwind]

Mentioned in https://codapi.org/faq/, seems good for if the fiddle is down for some reason.
This is also similar to an idea by @/mal to run the examples at docs build time to ensure they at least run, even if they aren't tested.

[Akeboshiwind]

The way I was thinking of getting this working was:

• Parse mdx files using rehype or something (not ideal tbh)
• Get the inputs (ignoring e.g. the Text input)
• Generate all combinations of values and run the fiddle for all those
• Store in a global variable somewhere on the page

Not sure if this would get too big for some fiddles 🤔