- Re-enable docs presbumit check (checks if there has been direct
edits of /docs).
- Fix script to download and install mdbook
- Update Materials.md.html and re-run the build script
(build/run.py).
We add a bash script to download and install mdbook. For
linux or mac x86_64, we download prebuilts from github.
Otherwise, we use `cargo install` to build it with rust.