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.
- Use mdbook to generate a book from collection of documents
- documents are
- copied and processed from READMEs
- original content stored in the docs_src folder
- markdeep docs transformed into embedded html
- Main logic is in script docs_src/build/run.py