Add opt-in documentation discovery (llms.txt) to the MCP server

[rohitkumarbhagat]

Summary

Add an optional Data Commons documentation resource backed by
https://docs.datacommons.org/llms.txt.

When enabled, the MCP server:

• Exposes the current documentation index as an MCP resource.
• Adds a short server-level routing hint for API, client library, schema,
  dataset coverage, concept, and integration questions.
• Keeps statistical data questions routed through the existing MCP tools and
  research skills.

The existing server.md remains unchanged from upstream.

Why

The MCP tools work well for statistical research, but agents also receive
questions about Data Commons APIs, client libraries, schema, datasets, and
integrations. Without a documentation discovery path, clients may answer from
memory or perform broad web searches.

Using a resource keeps the documentation out of the always-loaded server
instructions. The complete index is fetched only when a client reads the
resource, and the index can then direct the client to the relevant official
documentation pages.

The resource is disabled by default so Custom Data Commons deployments do not
automatically receive base Data Commons documentation guidance.

Rollout

DC_ENABLE_DOCUMENTATION_RESOURCE controls both resource registration and the
routing hint.

It defaults to false and is initially enabled only in the autopush
environment. Staging and production remain disabled while the behavior is
evaluated.

Validation

• All 45 Data Commons MCP tests pass.
• Ruff passes.
• Cloud Build deployment YAML parses successfully.
• A historical statistics test used the existing research skill and tools
  without reading the documentation resource.
• A documentation test read the resource once, avoided downloading the index
  a second time, and generated correct Python V2 client code from official
  documentation.
• server.md is byte-identical to the current upstream version.

[gemini-code-assist]

Code Review

This pull request introduces the option to expose the official Data Commons documentation resource, controlled by the new DC_ENABLE_DOCUMENTATION_RESOURCE setting. When enabled, the server registers a documentation index resource and appends a routing hint to the server instructions. The feedback recommends moving the requests import from the module level to inside the registration function to defer loading and keep server startup fast when the feature is disabled.