Update ADK doc according to issue #1256 - 2

[adk-bot]

2. Summary of the change

Doc file: docs/evaluate/index.md

Current state:

(Does not mention Agent Simulator or Tool Simulation)

...

User Simulation

...

Proposed Change:

Add a new section "Tool Simulation" or "Agent Simulator" linking to the new page.

Tool Simulation / Agent Simulator

When evaluating agents, it is often useful to simulate tool outputs rather than calling real APIs. This allows for:

• Deterministic testing
• Fault injection (errors, latency)
• Cost savings

The Agent Simulator provides a flexible way to mock tools and inject behaviors.

Reasoning:
Users looking at evaluation docs should be aware of the new Agent Simulator capability for mocking tools during evaluation.

Reference: src/google/adk/tools/agent_simulator/agent_simulator_engine.py

[joefernandez]

duplicate of: #1259