feat: Mdbook, use `rustdoc_include` and anchors

[julio4]

Related to #70

We can replace include with rustdoc_include in the mdbook files. This allows readers to expand the contract and view the tests directly in the book (without altering the current appearance).

Todo:

• Move all tests into the same contract files
• Adjust listing inclusions in book files
• Insert contract/test anchors as necessary
• Revise the Contribution guidelines to reflect these changes

An example will be provided soon...