Adds rustdocs to more of the public API

[edmorley]

Currently there are many public types and APIs that have no rustdocs.

We should make sure most of these are documented, to both make the libcnb-specifics easier to understand/use, but also to reduce the number of times someone writing a buildpack has to look up what a CNB concept means by referring back to the CNB spec.

We may also want to consider sprinkling #![warn(missing_docs)] in some of the libcnb crates to try and prevent regressions (with the understanding that it's fine to add corresponding allows in cases where no docs is fine).

[edmorley]

We may also want to consider sprinkling #![warn(missing_docs)] in some of the libcnb crates to try and prevent regressions

Plus also #![warn(missing_doc_code_examples)] - would help prevent things like #406.

[edmorley]

#441 helps improve things significantly here for libcnb-test.