docs: add XML documentation for HQC key parameters (Batch 13b-2)

[KonradSop]

Description

Adds XML documentation to the HQC KEM parameter and key classes (all previously undocumented):

• HqcParameters — class summary, the hqc128 / hqc192 / hqc256 instances, and the public size/length properties (EncapsulationLength, SessionKeySize, Name, PublicKeyBytes, SecretKeyBytes, SecretLength).
• HqcKeyParameters — base class summary, Parameters.
• HqcKeyGenerationParameters — class summary, constructor (with <exception>), Parameters.
• HqcPublicKeyParameters / HqcPrivateKeyParameters — class summaries, constructors, GetEncoded, GetPublicKey / GetPrivateKey, and the obsolete PublicKey / PrivateKey properties.

PQC reference style preserved — documentation only, no code changes.

Key Accomplishments

• HQC discoverability: IDE tooltips now cover the full public parameter/key surface (0 → docs across 5 files).
• Consistent style: matches the merged ML-KEM/ML-DSA parameter docs (Batch 5/9).

Verification

• Build Status: dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release — 0 errors, no new warnings.
• Scope: Documentation-only; no behavioural or signature changes.

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

See also Contributing Guidelines.