docs: Update llm-agents with built in planner example.md

[kanlanc]

Created a draft version for the issue #173

@wietsevenema , @hangfei would love your thoughts on should be added or removed to this, I added some comments on the issue directly about adding validation on the models that support the thinking config itself.

Finally, if there is an ideal number on what should be the thinking budget usually

Some things I wasn't too clear on my lack of creativity with what would be an ideal situation to demonstrate the planner capabilities.

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[kanlanc]

@wietsevenema , @hangfei , @boyangsvl

Hey guys,

Would love some direction for the above questions so I can move forward more on this

[lavinigam-gcp]

@kanlanc Hey Vikranth, sorry for the delay. I will be reviewing your PR and help you get this merged. :)

[lavinigam-gcp]

@kanlanc I have requested few changes, and I have tested the updated code at my end. Once fixed, will merge. Thank you again for taking time to add this example! :)

[Sita04]

Hi @kanlanc,

Just fyi, I've made changes to the Planner doc to include info on BuiltInPlanner and PlanReActPlanner. But you can still go ahead and modify the "Full Example" and merge this PR.

[joefernandez]

BLOCKED due to commit message compliance.