Update ADK doc according to issue #1173 - 5 - Deploy with Express Mode

[adk-bot]

5. Summary of the change

Doc file: docs/deploy/agent-engine/deploy.md

Current state:

The documentation for deploying to Agent Engine does not mention support for Vertex AI Express Mode (using an API key).

Proposed Change:

Add a section or note about deploying with Express Mode.
Mention the --api_key option for adk deploy agent_engine.
Explain that this initializes vertexai with the API key instead of project/location credentials.

Reasoning:
The CLI now supports deploying to Agent Engine with an API key (Express Mode), enabling usage without full GCP project credentials setup in some cases. This option should be documented.

Reference: src/google/adk/cli/cli_deploy.py

[github-actions]

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