docs: add XML documentation for NTRU key parameters (Batch 13b-1)

[KonradSop]

Description

Adds XML documentation to the NTRU KEM parameter and key classes:

• NtruKeyParameters — base class summary, Parameters, abstract GetEncoded.
• NtruKeyGenerationParameters — class summary, constructor, GetParameters.
• NtruPublicKeyParameters — class summary, FromEncoding factory (with <param>/<returns>/<exception>), GetEncoded, and the obsolete PublicKey property.

NtruParameters was already documented; this fills the remaining gaps. PQC reference style preserved — documentation only, no code changes.

Key Accomplishments

• NTRU discoverability: IDE tooltips now cover the public key/parameter surface.
• Accurate exception contract: documented on FromEncoding where the constructor validates its arguments.
• 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.