docs: explain publishing community servers to the MCP Registry

[CryoThrust]

Summary

Addresses #4171 by documenting the recommended path for community-built MCP servers:

• Directs community projects to the MCP Registry instead of this reference-server repository.
• Lists the artifact, server.json, validation, authentication, and publication steps.
• Documents the GitHub namespace requirement and links to the Registry's current quickstart.
• Clarifies that reference-server issues should continue to be reported here.

Verification

• git diff --check
• README assertions for the Registry commands and namespace