[core][docs] Clarify Ray Core walkthrough benchmark setup

[kalyanamdewri]

Summary:

• Clarifies that standalone copies of the Ray Core walkthrough benchmark should call ray.init() before starting the timer.
• Related to [Core] Core gentle walkthrough example doesn't show the benefit of Ray. #40653.

Validation:

• python3 -m json.tool doc/source/ray-core/examples/gentle_walkthrough.ipynb >/tmp/gentle_walkthrough.jsoncheck — passed
• git diff --check — passed

[gemini-code-assist]

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[kalyanamdewri]

/gemini review

[gemini-code-assist]

Code Review

This pull request updates the gentle_walkthrough.ipynb notebook to add a note explaining that ray.init() should be called before starting the timer if the benchmark code is copied into a separate script, ensuring task execution is measured instead of cluster startup. There are no review comments, so I have no feedback to provide.