13. Secret Manager Integration

Doc file: docs/integrations/secret-manager.md

Current state:

The documentation advises users to use Google Cloud Secret Manager for storing credentials, but there is no documentation on the new built-in ADK SecretManagerClient.

Proposed Change:

Create a new integration page for Google Cloud Secret Manager. Explain that ADK now includes a SecretManagerClient (google.adk.integrations.secret_manager.secret_client) which simplifies retrieving secrets. Provide an example of how to instantiate it (using ADC, an auth token, or a service account JSON string) and how to call get_secret(resource_name) to securely fetch API keys or tokens for use with ADK tools.

Reasoning:
This is a new official integration (mentioned in the release summary) that makes it much easier for developers to securely load credentials in production deployments without having to manually set up the standard GCP SDK clients.

Reference: src/google/adk/integrations/secret_manager/secret_client.py