Update ADK doc according to issue #1173 - 4 - CLI adk eval_set

[adk-bot]

4. Summary of the change

Doc file: docs/evaluate/index.md

Current state:

The documentation mentions adk eval but not the adk eval_set commands for creating and managing eval sets via CLI.

Proposed Change:

Add a section "Managing Eval Sets via CLI" (or similar) describing the new commands:

• adk eval_set create: Creates an empty EvalSet.
• adk eval_set add_eval_case: Adds eval cases to an eval set, e.g., from a conversation scenarios file.

Mention that these commands allow programmatic creation of evaluation datasets.

Reasoning:
New CLI commands adk eval_set create and adk eval_set add_eval_case allow users to manage evaluation datasets from the command line, complementing the web UI approach.

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

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