Docs: Add guidance for using .env and load_dotenv() with Python

[Muhammad-Talha33]

Problem
The current "Hello World" example in the docs assumes that the OPENAI_API_KEY
is already available in the system environment variables.

However, many Python developers store keys in a .env file.
If they don't call load_dotenv(), they run into the following error:

openai.OpenAIError: The api_key client option must be set either by passing api_key to the client or by setting the OPENAI_API_KEY environment variable

Suggested improvement
Add a note in the docs for Python users that if you are using a .env file,
you need to load it with:

from dotenv import load_dotenv
load_dotenv()

before creating the agent or runner.

Benefit
This will save beginners from confusion and make the SDK easier to adopt
for Python users following common .env practices.

[seratch]

Thanks for writing in. I don't think this is specific to this SDK and it's more of a general knowledge. Thus, we won't have the guide for it.