Docs: Address text-davinci-003 deprecation and new API structure in NewsAgent example

[kumarrah2002]

What does this PR do?

When testing the NewsAgent example in the README, there are multiple issues with the current implementation provided due to deprecation. The following changes have been made:

1. OpenAI has changed the client functionCompletion to completions.
2. The model text-davinci-003 has been deprecated and needs to be updated with a current model (I used gpt-3.5-turbo but it can be changed to any model in the OpenAI fleet)
3. Retrieving the LLM response has to be done with llm_response.choices[0].message.content.strip() now.

These changes allow for seamless user experience when following the docs to make their own agent.

Sorry for the spam, I unintentionally pushed my new changes for a different problem to the wrong branch, hence the revert.

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• [✅] Did you read the contributor guideline, Pull Request section?
• [✅] Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86%. Comparing base (145daea) to head (814e330).
Report is 1 commits behind head on main.

Additional details and impacted files

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  Files        37     37           
  Lines      2557   2557           
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  Hits       2191   2191           
  Misses      366    366           

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