1. Create new documentation for the Agent Identity integration

Doc file: docs/integrations/agent-identity.md

Current state:

(File does not exist)

Proposed Change:

Create a new integration page for Agent Identity.
Add catalog frontmatter (e.g. catalog_title: Agent Identity, catalog_description: Manages the complete lifecycle of an access token using the GCP Agent Identity Credentials service.).
Include installation instructions: pip install "google-adk[agent-identity]".
Show how to register the provider globally:

from google.adk.auth.credential_manager import CredentialManager
from google.adk.integrations.agent_identity import GcpAuthProvider

CredentialManager.register_auth_provider(GcpAuthProvider())

Show how to use it with toolsets (like McpToolset) by defining the scheme:

from google.adk.integrations.agent_identity import GcpAuthProviderScheme
auth_scheme = GcpAuthProviderScheme(name="my-jira-auth_provider")
mcp_toolset_jira = McpToolset(..., auth_scheme=auth_scheme)

Reasoning:
The new Agent Identity integration (GcpAuthProvider) was added to the Python SDK to handle authentication against GCP's Agent Identity Credentials service, but it lacks documentation. Creating this integration page will expose it in the tools catalog.

Reference: src/google/adk/integrations/agent_identity/gcp_auth_provider.py, src/google/adk/integrations/agent_identity/README.md