digest: improve documentation and test examples in readme

Lack of readme example tests has caused #1072.

Also we probably should unify documentation from the hashes and digest' readmes. I think of having most of docs in the digest's readme, which will be included into digest's docs, and we will reference those examples from the hashes readme and hash implementation crate-level docs.

Including working hash tests into digest's docs will cause a weird dependency cycle digest <- sha2 <- digest, but I think it should work since sha2 and other crates will be dev-dependencies.

Compare the hashes README, the digest README, and the hash implementation crate-level documentation to identify duplicated examples and documentation. Start by checking how README content is included in digest's docs and how dev-dependencies affect the proposed digest <- sha2 <- digest cycle. Done means unified documentation with working hash example tests referenced consistently across those locations.