docs: add XML documentation for modern-curve parameters (Batch 7)

[KonradSop]

Description

Adds XML documentation to the public/private/key-generation parameter classes for the RFC 8032 EdDSA curves (Ed25519, Ed448) and RFC 7748 X-DH curves (X25519, X448). These are user-facing entry points for modern signing and key-agreement workflows that previously carried no API documentation.

• Ed25519 / Ed448 public-key params — class summary citing RFC 8032; per-constructor exception contracts (length validation, EOF on stream read, point-decode failures); Encode/GetEncoded summaries; Verify documenting the pure / context / prehash variant selector and the conditions each places on ctx and msgLen.
• Ed25519 / Ed448 private-key params — class summary noting the lazy public-key cache; constructor exceptions; Encode/GetEncoded; GeneratePublicKey; Sign mirroring the Verify documentation.
• X25519 / X448 public-key params — class summary citing RFC 7748 and noting that point validation is deferred to scalar multiplication; constructor exceptions; Encode/GetEncoded.
• X25519 / X448 private-key params — class summary noting clamped scalar; GeneratePublicKey; GenerateSecret documenting the InvalidOperationException raised on degenerate (all-zero) secrets.
• Key-generation parameters — class and constructor summaries citing the relevant RFC and noting the fixed strength.

Key Accomplishments

• Discoverability: First-time IDE tooltips for the modern curves now identify the RFC, key/seed/scalar size, and signature size.
• Accurate exception contracts: <exception> tags only added where the method body actually throws — the EdDSA Verify/Sign algorithm-selector exceptions, X-DH agreement failure, length-validation guards, and stream-EOF guards.
• Consistent style: Class summaries follow the Batch 5 PQC-params pattern (RFC reference, what is wrapped, validation strategy). Span-overload entries cross-reference the byte-array overload via <see cref=...>.

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.