### What
This PR changes setup for the turbopack documentation.
Long story in short, we want to 2 types of documentation: one for high
level description (archtecture, etcs) and another for the api doc.
rustdoc / cargo doc unfortunately doesn't support to integrate both
easily.
PR adds mdbook setup to workaround those - instead of using rustdoc's
index as entrypoint, mdbook becomes an entrypoint to rustdoc. This
allows to host plain text document, and also able to access full rustdoc
pages.
preview: https://turbopack-rust-docs-b6qta4xdq.vercel.sh/
rustdoc:
https://turbopack-rust-docs-b6qta4xdq.vercel.sh/rustdoc/index.html
Closes PACK-2475
Closes PACK-2468
### What
We'd like to deploy generated rustdocs from turbopack & related packages
(next-swc), setting up a CI workflow to automate those.
Since we need next.js side packages to build docs, this CI sets up
things in this repo.
Closes PACK-2375.