docs: add XML documentation for SLH-DSA parameters (Batch 8)

[KonradSop]

Adds XML documentation to the SLH-DSA parameter classes under crypto/src/crypto/parameters — the stateless hash-based signature scheme standardised by NIST in FIPS 205. Closes the post-quantum trio that PR #673 began (ML-KEM / ML-DSA), giving each of the three FIPS PQC primitives a consistent documentation surface.

• SlhDsaParameters — class summary citing FIPS 205 and explaining the pure / HashSLH-DSA split (FIPS 205 §10.2). Per-instance summaries identify each of the 24 parameter constants by NIST security category, fast/small variant and hash family. Public members (IsPreHash, Name, ParameterSet, ToString) documented with their semantic role.
• SlhDsaParameterSet — class summary describing the FIPS 205 tree-shape parameters. Per-constant summaries record the WOTS+, hypertree and FORS shape (n, h, d, k, a).
• SlhDsaKeyParameters — abstract base; class summary plus Parameters accessor doc.
• SlhDsaPublicKeyParameters — class summary explaining the (seed, root) tuple. FromEncoding documents the 2 * n length contract and the ArgumentNullException / ArgumentException it raises. GetEncoded documented.
• SlhDsaPrivateKeyParameters — class summary explaining the (SK.seed, SK.prf, PK.seed, PK.root) tuple. FromEncoding documents the 4 * n length contract. GetEncoded, GetPublicKey, GetPublicKeyEncoded documented.
• SlhDsaKeyGenerationParameters — class summary covering the SecureRandom + parameter-set pairing and noting why strength = 0. Both constructors document their exception contracts.

Key Accomplishments

• Discoverability: First-time IDE tooltips for each of the 24 SLH-DSA parameter constants now identify the NIST category, fast/small variant and hash family without forcing a trip to the FIPS 205 spec.
• Accurate exception contracts: <exception> tags only added where the method body actually throws — ArgumentNullException / ArgumentException on the FromEncoding and OID-lookup paths.
• Consistent style: Class summaries follow the Batch 5 ML-KEM / ML-DSA pattern (FIPS reference, what is wrapped, decoding contract). Internal members and pre-existing implementation comments left untouched.

Verification

• Build Status: dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release — succeeded on net6.0, netstandard2.0, net461 with 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.