docs: add XML documentation for modern asymmetric key-pair generators (Batch 10)

[KonradSop]

Adds XML documentation to the seven key-pair generators that pair with the parameter classes covered by PRs #673 (Batch 5), #677 (Batch 7), #678 (Batch 8) and #679 (Batch 9):

• MLKemKeyPairGenerator — class summary citing FIPS 203; Init documents the InvalidCastException raised when the supplied KeyGenerationParameters is not an MLKemKeyGenerationParameters; GenerateKeyPair notes the SeedAndEncoding format used for the produced private key.
• MLDsaKeyPairGenerator — same shape as ML-KEM, citing FIPS 204.
• SlhDsaKeyPairGenerator — class summary citing FIPS 205; GenerateKeyPair notes that the three n-byte seeds (SK.seed, SK.prf, PK.seed) are drawn from the SecureRandom and that the hypertree root is computed up front.
• Ed25519KeyPairGenerator / Ed448KeyPairGenerator — class summaries citing RFC 8032; Init / GenerateKeyPair documented; class summary notes the seed size (32 / 57 bytes).
• X25519KeyPairGenerator / X448KeyPairGenerator — class summaries citing RFC 7748; Init / GenerateKeyPair documented; class summary notes the clamped scalar size (32 / 56 bytes).

Key Accomplishments

• Closes the modern-asymmetric documentation surface: with KeyGenerationParameters already documented (Batches 5/7/8/9 and docs: add XML documentation for ML-KEM and ML-DSA parameter gaps (Batch 9) #679), the matching IAsymmetricCipherKeyPairGenerator implementations now carry the same level of XML coverage.
• Accurate exception contracts: <exception cref="InvalidCastException"/> only added on the three PQC generators whose Init performs the cast; the four EdDSA/X-DH generators only capture SecureRandom so they don't throw.
• Build hygiene: added the missing using System; to the three PQC generator files so the new InvalidCastException cref resolves cleanly. No other source changes.
• Consistent style: Class summaries follow the same FIPS / RFC reference pattern used in Batches 5/7/8/9.
• No overlap with open PRs: scope is disjoint from feature/modern-curves-docs (Batch 7), feature/slh-dsa-docs (Batch 8) and feature/ml-pqc-gaps-docs (Batch 9). The recent 5cdc6f51 Refactor Falcon key parameter classes upstream commit does not touch any of these files.

Verification

• Build Status: dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release — succeeded on net6.0, netstandard2.0, net461 with no new warnings (the remaining warnings are pre-existing in unrelated files).
• Scope: Documentation-only; the only non-doc edits are three using System; directives needed for the InvalidCastException cref.

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.