feat: MDBook `keeper` tests

[Autoparallel]

Right now we have a commented out preprocessor for the MDBook:

# TODO (autoparallel): there's a bunch to fix with this
# [preprocessor.keeper]
# command     ="mdbook-keeper"
# manifest_dir="."
# externs     =["ronkathon", "rand"]

If we uncomment this, code snippets in the book will run and be checked like doctests, which is nice. However, many of these are broken as is.

To replicate:
Make sure you:

cargo install mdbook
cargo install mdbook-keeper

Then from the root:

mdbook build

[mrdaybird]

@Autoparallel Suppose if we fixed the code snippets in the book, I was wondering how are we supposed to make them work on the web, since mdbook sends the code to rust playground which does not have the ronkathon crate.

I see that an open PR (rust-lang/mdBook#2503) in mdbook but it is kind of stale....

Any other ideas?

[Autoparallel]

We will create our own playground :)

[Autoparallel]

@mattes would you want to meet up sometime and deploy our own playground to use with ronkathon?

[mattes]

Happy to help.

[Autoparallel]

Alright -- let's revisit this sometime soon. Could be a fun endeavor!