sha3: improve hasher usage documentation

[weifanglab]

Summary

• Link the SHA-3 and Keccak hasher types to the Digest trait so their usage is discoverable from the generated API documentation.
• Update the README example to demonstrate both the one-shot digest interface and incremental hashing with multiple update calls.
• Correct the Sha3_512 documentation, which incorrectly described it as a SHA-3-256 hasher.

Closes #441.

Testing

• cargo +1.85.0 test -p sha3 --all-features --locked
• cargo test -p sha3 --all-features --locked
• cargo doc -p sha3 --all-features --no-deps --locked
• cargo fmt --all -- --check
• git diff --check