fix(dashboard): link relay connector docs

[xuezhaolan]

Summary

• Adds the existing Relay messaging docs URL to the Relay channel metadata.
• Covers the metadata link with a small regression test that also verifies the docs page exists.

Closes #78373

Test Plan

• python -m pytest tests/hermes_cli/test_relay_platform_docs_url.py -q -o 'addopts='
• python -m py_compile hermes_cli/web_server.py tests/hermes_cli/test_relay_platform_docs_url.py
• git diff --check
• Independent Codex diff review: no security or logic issues found