docs: add AI Manifest + WellKnownAI as optional references (discoverability + snapshots)

[agentstandards]

Adding two optional references for service/agent discovery and auditability:

• AI Manifest — optional reference for /.well-known/ai.json + OpenAPI/JSON Schema discovery (with MCP/agents.json mapping). https://ai-manifest.org
• WellKnownAI — registry/spec examples and public snapshots (no PII, mirroring allowed). https://wellknownai.org

Non-binding, community-driven; alphabetical order respected where applicable.

[agentstandards]

Hi This adds one neutral “optional reference” in See also (alphabetical). No main‑text changes. Happy to adjust/move if you prefer. Thanks!

[agentstandards]

Hi maintainers, gentle ping on this PR. This adds two optional references for service/agent discovery and auditability (non-binding, alphabetical order respected).

• AI Manifest — optional reference for /.well-known/ai.json + OpenAPI/JSON Schema discovery (with MCP/agents.json mapping).
• WellKnownAI — registry/spec examples and public snapshots (no PII, mirroring allowed).

Happy to adjust section title (Links/Resources/See also) or wording per your repo style. Thanks!
#163