add asciinema output

[zimbatm]

for long-running processes, it might be nice to record the output, and have a "player" to replay it, instead of having it inline in the document.

something like asciinema

[phaer]

Just in case you don't know it already, there's https://github.com/Byron/termbook/ which supports the creation of asciinema recordings from markdown code blocks.
It's a plugin for https://rust-lang.github.io/mdBook/ and uses cmark-pulldown as a parser and https://github.com/Byron/pulldown-cmark-to-cmark/tree/main to serialize the AST back into markdown.

I've stumbled upon it while playing around with related ideas in https://github.com/phaer/mdbook-nix/ .