docs: add XML documentation for ML-KEM and ML-DSA parameters (Batch 5)

[KonradSop]

Describe your changes

Adds XML documentation for the public API surface of the ML-KEM (FIPS 203) and ML-DSA (FIPS 204) parameter classes. This is Batch 5 of the ongoing documentation effort, following the pattern established in merged PRs #666, #669 and the in-review PRs #670, #671.

Files covered:

• MLKemParameters — parameter set constants (ml_kem_512/768/1024) + Name, ParameterSet, ToString.
• MLKemPublicKeyParameters — class summary, FromEncoding, GetEncoded.
• MLKemPrivateKeyParameters — class summary, Format enum (incl. per-value descriptions), FromEncoding, both FromSeed overloads, GetEncoded, GetPublicKey, GetPublicKeyEncoded, GetSeed, PreferredFormat, WithPreferredFormat.
• MLDsaParameters — pure and HashML-DSA-with-SHA512 variants, IsPreHash, Name, ParameterSet, ToString.
• MLDsaPublicKeyParameters — class summary, FromEncoding, GetEncoded.

No behavioural changes. References to FIPS 203 / FIPS 204 are included where it clarifies semantics.

How has this been tested?

dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release succeeds with 0 errors across all target frameworks (net6.0, netstandard2.0, net461). No new tests — this is a documentation-only change.

Checklist before requesting a review

• I have performed a self-review of my code
• I have kept the patch limited to only change the parts related to the patch
• This change requires a documentation update

[peterdettman]

Thanks for the PR. Rebased and merged.