Add the `bindgen` Users Guide

This commit adds a Users Guide, built with [mdbook][] (the same tool used
by *The Rust Book*).

It moves all the user documentation from the README.md into the Users Guide. It
leaves the README.md as a splash page featuring an example of what `bindgen` is
and does, a link to the Users Guide, a link to the docs.rs/bindgen API reference
documentation, and a link to CONTRIBUTING.md.

It ensures that the Users Guide builds with `mdbook`, and all code snippets
within it build as doctests and pass, on every CI run.

Finally, it builds and uploads the Users Guide every time CI passes on the
master branch.

[mdbook]: https://github.com/azerupi/mdBook

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