Move playground within the website

[max-sixty]

As discussed on Discord:

It would be less of a jump, and so fewer people giving up exploring the language, if Playground could share a header and formatting with the rest of the site.

I don't think it can't be build purely with Hugo given it requires wasm & JS, but it could be copied into a path as part of a build step (as we do in the build).

It would require adjusting the Playground's CSS to be more similar to the site's.

This was originally inspired by @alonso's Rulex. Though it looks like they've recently regressed a bit and it's not exactly the same header.

[mklopets]

The easiest way would be just to use an iframe here. Ideally, this would have a maximize button, though (which is why I didn't immediately open a PR)

[mklopets]

FWIW: I think this same issue also applies to the "book"!

[mklopets]

Opened a PR to move both 🤷