This updates the crate-level docs to add a little more detail for each crate. This also drops the `mdbook` library crate, as it is no longer needed.
This sets up mdbook-preprocessor with the intent of being the core library that preprocessors use to implement the necessary interactions.
These are two new crates intended to support implementing preprocessors and renderers. Currently these stubs just have MDBOOK_VERSION, but future commits will migrate more code to these crates.
