Update ADK doc according to issue #1048 - 1 - Interactions API support

[adk-bot]

[Feature] Add documentation for the new Interactions API support.
The newly released Gemini Interactions API is now supported in ADK. A new page should be created to document this feature.

Proposed Change:
Create a new file docs/agents/interactions-api.md with the following content:

# Using the Gemini Interactions API

The Gemini Interactions API provides stateful conversation capabilities, allowing you to chain interactions using a `previous_interaction_id` instead of sending the full conversation history with each request. This can be more efficient for long conversations.

To enable the Interactions API, set the `use_interactions_api=True` parameter in the `Gemini` model configuration.

## Example

```python
from google.adk.agents.llm_agent import Agent
from google.adk.models.google_llm import Gemini

root_agent = Agent(
    model=Gemini(
        model="gemini-2.5-flash",
        use_interactions_api=True,
    ),
    name="interactions_test_agent",
    description="An agent for testing the Interactions API integration",
    instruction="You are a helpful assistant.",
)

For a more detailed example, see the Interactions API sample.

Also, add a link to this new page in the `mkdocs.yml` file under the "Agents" section.

**Reasoning**:
This is a major new feature that provides a new way to interact with Gemini models. The documentation should explain what it is, how to use it, and provide an example.

**Reference**:
- `src/google/adk/models/google_llm.py`
- `src/google/adk/flows/llm_flows/interactions_processor.py`
- `contributing/samples/interactions_api/`