docs: remove Agent Config page

[haranrk]

Summary

• Removes the Agent Config documentation page (docs/agents/config.md), served at https://adk.dev/agents/config/
• Removes its navigation entry from mkdocs.yml

Notes

The following files still contain links to /agents/config/ and are not modified by this PR (they belong to other sections whose narrative depends on the Agent Config concept):

• docs/visual-builder/index.md (3 links)
• tools/feature-matrix/start.md (1 link)

Let me know if these references should also be cleaned up.

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[koverholt]

Is this related to the changes in google/adk-python@44cd1167 and google/adk-python@d7d73174? Those commits deprecate the Agent Config classes and the from_config loader, but the deprecation messages state that "config is now loaded via reflection so the separate config class is no longer needed", which seems like an internal refactor rather than a removal of the YAML feature. Users can still define an Agent Config YAML, and the adk web Visual Builder depends on it.

Given that, it seems only the run-programmatically section is affected since it documents the deprecated config_agent_utils.from_config. Can you confirm whether the intent of those changes is to remove the entire Agent Config page, or just update the affected subsection?