Enable crate feature info in rustdocs

[clehner]

As mentioned in #360 (comment)

The doc_cfg and doc_auto_cfg features are described in rust-lang/rust#90502 and enable rustdocs to render uses of #[cfg(feature = ...]). These features are enabled using a cfg attribute docsrs which is enabled in Cargo.toml for docs.rs. Since doc_cfg and doc_auto_cfg are unstable features, this docsrs attribute is left disabled by default. It can also be enabled on the command-line as in the following command to build the docs using a nightly toolchain (Run rustup update nightly first if you haven't installed nightly recently):
RUSTDOCFLAGS='--cfg docsrs' cargo +nightly doc --no-deps --features secp256r1,secp256k1,ripemd-160,http-did

After building docs with that command, open target/doc/ssi/hash/fn.sha256.html:

Features secp256r1,secp256k1,ripemd-160,http-did are set as enabled by default for the docs.rs build (from 883f3c4 in #311).

I combined the two sha256 function implementations into one function, as I wasn't otherwise able to get the docs to list its supported crate features as "sha2 or ring". (Similar: rust-lang/rust#90497 (comment))