Update ADK doc according to issue #1438 - 13

[adk-bot]

13. Document the --api_key option for adk deploy agent_engine

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

Current state:

The following example deploy command uses the multi_tool_agent sample code as
the project to be deployed:

PROJECT_ID=my-project-id
LOCATION_ID=us-central1
GCS_BUCKET=gs://MY-CLOUD-STORAGE-BUCKET

adk deploy agent_engine \
        --project=$PROJECT_ID \
        --region=$LOCATION_ID \
        --staging_bucket=$GCS_BUCKET \
        --display_name="My First Agent" \
        multi_tool_agent

Proposed Change:

The following example deploy command uses the multi_tool_agent sample code as
the project to be deployed:

Option 1: Deploy with Google Cloud Project (Standard)

PROJECT_ID=my-project-id
LOCATION_ID=us-central1
GCS_BUCKET=gs://MY-CLOUD-STORAGE-BUCKET

adk deploy agent_engine \
        --project=$PROJECT_ID \
        --region=$LOCATION_ID \
        --staging_bucket=$GCS_BUCKET \
        --display_name="My First Agent" \
        multi_tool_agent

Option 2: Deploy with API Key (Express Mode)

If you are using Express Mode, you can deploy using your API Key:

adk deploy agent_engine --api_key=YOUR_API_KEY my_agent

Reasoning:
The adk deploy agent_engine command now supports an --api_key argument for Express Mode deployment. This option should be documented alongside the standard Google Cloud Project deployment method.

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

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