docs: Add comprehensive SDK documentation and README improvements

[dasiths]

Summary

Adds complete SDK documentation (22 markdown files) covering all AAuth .NET SDK workflows and signing modes, plus README improvements.

Documentation (docs/)

• Foundation: Index, getting started, protocol concepts (four participants: Agent, Resource, PS, AS)
• Signing Modes: Overview + individual guides for hwk, jwks_uri, jwt, jkt-jwt
• Workflows: Identity-based, resource-managed, PS-asserted, federated, bootstrap/enrollment, deferred consent
• Server Implementation: Verification middleware, resource metadata, token issuance, replay detection, multi-scheme verification
• Advanced: Missions, platform attestation, key management, error handling
• Reference: Full configuration options for all SDK components

All docs link to the AAuth Explorer as the golden reference.

README Changes

• Renamed project to AAuth SDK for .NET
• Added 🚧 draft-spec notice at the top
• Removed outdated WIP/Phase 3 status banner
• Moved per-sample running instructions to samples/README.md
• Consolidated repository layout table
• Corrected the four protocol participants (Agent, Resource, PS, AS — AP is a supporting role)

Prior Implementation (included in branch)

This branch also includes the gap remediation implementation work:

• All four signing modes end-to-end (hwk, jwks_uri, jwt, jkt-jwt)
• Spec-compliant error model (supported_algorithms on unsupported_algorithm errors)
• MockAgentProvider sample
• Demo mode constraints (jwt requires PS, identity uses hwk/jwks_uri)
• Verification hardening and extensibility seams

All 320 tests pass (130 unit + 190 conformance).