Found docs updates needed from ADK python release v1.19.0 to v1.20.0

I have analyzed the code changes between ADK Python releases v1.19.0 and v1.20.0 and identified the following documentation updates.

Compare link: https://github.com/google/adk-python/compare/v1.19.0...v1.20.0

### 1. New `TransferToAgentTool` and Unique Sub-Agent Name Validation
- **Summary of Change:** The `transfer_to_agent` tool now has a `TransferToAgentTool` wrapper that provides enum constraints to the `agent_name` parameter, preventing the LLM from using non-existent agent names. Additionally, the framework now validates that all sub-agents have unique names.
- **Files to Update:**
- `docs/agents/multi-agents.md`
- `docs/tutorials/agent-team.md`
- **Instructions:**
- In `docs/agents/multi-agents.md`, update the section on LLM-Driven Delegation to mention the `TransferToAgentTool` and its benefits.
- Add a note about the unique name validation for sub-agents in `docs/agents/multi-agents.md`.
- In `docs/tutorials/agent-team.md`, refactor the examples to use the `TransferToAgentTool` instead of the raw `transfer_to_agent` function.
- **Reference:** `src/google/adk/tools/transfer_to_agent_tool.py`, `src/google/adk/agents/base_agent.py`

### 2. Streaming Function Call Arguments
- **Summary of Change:** The ADK now supports streaming for function call arguments, allowing for more responsive user experiences.
- **Files to Update:**
- `docs/streaming/dev-guide/part3.md`
- **Instructions:**
- Add a new section to `docs/streaming/dev-guide/part3.md` explaining the new feature of streaming function call arguments.
- Provide a code example of how to use this feature.
- **Reference:** `src/google/adk/utils/streaming_utils.py`

### 3. Multi-Provider Support for Claude and LiteLLM
- **Summary of Change:** The ADK now has built-in support for Anthropic's Claude models and a wide range of other models through LiteLLM.
- **Files to Update:**
- `docs/agents/models.md`
- `docs/tutorials/agent-team.md`
- **Instructions:**
- Update `docs/agents/models.md` to include sections for Claude and LiteLLM, explaining how to configure and use them.
- Update the `agent-team.md` tutorial to include an example of using a Claude or LiteLLM model in one of the agents.
- **Reference:** `src/google/adk/models/anthropic_llm.py`, `src/google/adk/models/lite_llm.py`

### 4. `APIRegistryToolset`
- **Summary of Change:** A new `APIRegistryToolset` allows agents to discover and use tools from a Google Cloud API Registry.
- **Files to Update:**
- `docs/tools-custom/index.md`
- Create `docs/tools-custom/api-registry-toolset.md`
- **Instructions:**
- Add a link to the new `api-registry-toolset.md` page in `docs/tools-custom/index.md`.
- Create a new page `docs/tools-custom/api-registry-toolset.md` with detailed documentation on how to use the `APIRegistryToolset`, including setup and examples.
- **Reference:** `src/google/adk/tools/api_registry.py`

### 5. `AgentTool` Plugin Propagation
- **Summary of Change:** The `AgentTool` now has an `include_plugins` parameter to control whether plugins from the parent runner are propagated to the agent's runner.
- **Files to Update:**
- `docs/tools-custom/function-tools.md`
- **Instructions:**
- In `docs/tools-custom/function-tools.md`, update the `AgentTool` section to document the `include_plugins` parameter and its usage.
- **Reference:** `src/google/adk/tools/agent_tool.py`

### 6. New `/list-apps` Web Endpoint
- **Summary of Change:** The ADK web server now has a `/list-apps` endpoint to list available applications with details.
- **Files to Update:**
- Create `docs/web/reference.md`
- **Instructions:**
- Create a new file `docs/web/reference.md` to document the ADK web server's API endpoints.
- Add documentation for the `/list-apps` endpoint, including its parameters and response format.
- **Reference:** `src/google/adk/cli/adk_web_server.py`

### 7. Update to Python 3.10
- **Summary of Change:** The minimum required Python version for ADK has been raised to 3.10.
- **Files to Update:**
- `docs/get-started/python.md`
- **Instructions:**
- Update the Python version requirement in `docs/get-started/python.md` to 3.10.
- Search for and update any other mentions of the minimum Python version throughout the documentation.
- **Reference:** `src/google/adk/a2a/converters/part_converter.py`

### 8. Spanner Toolset updates
- **Summary of Change:** Added `vector_store_similarity_search` to `SpannerToolset`. `SpannerVectorStoreSettings` is introduced to support this feature.
- **Files to Update:**
- `docs/tools/built-in-tools.md`
- **Instructions:**
- Update the Spanner toolset documentation to include the new `vector_store_similarity_search` tool and the `SpannerVectorStoreSettings`.
- **Reference:** `src/google/adk/tools/spanner/search_tool.py`, `src/google/adk/tools/spanner/settings.py`