Composite sigs update

[feventura]

This PR include:

• Documentation requested by Missing Composite documentation #475
• Double OCTET STRING encoding for ED private key according with RFC 8410 pointed by The privateKey encoding for pure ml-dsa differs from the privateKey encoding for the ml-dsa part in composite ml-dsa-xxxx #466
• Some extra documentation updates that were missed when the composite code was migrated from "draft-ounsworth-pq-composite-sigs" to "draft-ietf-lamps-pq-composite-sigs"

[feventura]

Changed the logic on get_composite_idx to one that would not depend on the idx might be an issue if the two lists compared are not in sync, instead the search to get the OID is done by the algorithm name.
This would close #552 .