Found docs updates needed from ADK python release v1.29.0 to v1.31.0

Compare URL: https://github.com/google/adk-python/compare/v1.29.0...v1.31.0

### 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:
> ```python
> 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:
> ```python
> 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

### 2. **Create new documentation for the Parameter Manager integration**

**Doc file**: docs/integrations/parameter-manager.md

**Current state**:
> (File does not exist)

**Proposed Change**:
> Create a new integration page for Parameter Manager.
> Add catalog frontmatter (e.g. `catalog_title: Parameter Manager`, `catalog_description: A client for interacting with Google Cloud Parameter Manager.`).
> Show how to initialize the `ParameterManagerClient`:
> ```python
> from google.adk.integrations.parameter_manager.parameter_client import ParameterManagerClient
>
> # Using default credentials
> client = ParameterManagerClient()
>
> # Or with a service account json string
> client = ParameterManagerClient(service_account_json="...")
>
> # Or with an auth token
> client = ParameterManagerClient(auth_token="...")
> ```
> Explain how to retrieve a parameter:
> ```python
> value = client.get_parameter("projects/my-project/locations/global/parameters/my-param/versions/latest")
> ```

**Reasoning**:
A new ParameterManagerClient was introduced to fetch configuration and parameters securely from Google Cloud. Documenting it under integrations makes it discoverable for users building robust cloud-based agents.

**Reference**: src/google/adk/integrations/parameter_manager/parameter_client.py

### 3. **Mention GcpAuthProviderScheme in custom authentication docs**

**Doc file**: docs/tools-custom/authentication.md

**Current state**:
> The "Supported initial credential types" section lists API_KEY, HTTP, OAUTH2, OPEN_ID_CONNECT, and SERVICE_ACCOUNT, but does not mention the GCP Agent Identity provider.

**Proposed Change**:
> Add a mention of the GCP Agent Identity integration (`GcpAuthProviderScheme`) in the "Supported initial credential types" section or under a new section about Managed Authentication Services. Provide a link to the new `integrations/agent-identity.md` page. Mention that this provider automatically handles 2-legged and 3-legged OAuth flows using Google Cloud's IAM Connector Credentials service.

**Reasoning**:
The `GcpAuthProviderScheme` is a new, first-class custom auth scheme that provides managed credential flows. It should be mentioned in the primary authentication guide so developers know they don't have to build auth polling or consent completion entirely from scratch if using GCP.

**Reference**: src/google/adk/integrations/agent_identity/gcp_auth_provider.py