Update Model Client Docs to Mention API Key from Environment Variables

[victordibia]

Adds the following blurb to the model clients docs

API Keys From Environment Variables

In the examples above, we show that you can provide the API key through the api_key argument. Importantly, the OpenAI and Azure OpenAI clients use the openai package, which will automatically read an api key from the environment variable if one is not provided.

• For OpenAI, you can set the OPENAI_API_KEY environment variable.
• For Azure OpenAI, you can set the AZURE_OPENAI_API_KEY environment variable.

This is a good practice to explore, as it avoids including sensitive api keys in your code.

Why are these changes needed?

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.21%. Comparing base (676b611) to head (4492df3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5515       +/-   ##
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- Coverage   78.35%   60.21%   -18.15%     
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  Files         167      167               
  Lines        9926     9926               
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- Hits         7778     5977     -1801     
- Misses       2148     3949     +1801     

Flag	Coverage Δ
unittests	60.21% <ø> (-18.15%)	⬇️

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