Add a guide doc for GPTAssistantAgent

[IANTHEREAL]

Why are these changes needed?

As the OpenAI Assistant API evolves, the GPTAssistantAgent have also changed. We need a document that can show users how to configure and use the GPTAssistantAgent, rather than through multiple examples and simple code comments.

Closes #2465

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.

[IANTHEREAL]

I'm not quite sure how to fix this CI error. Should I upload the notebook to the website first? @ekzhu

[ekzhu]

@IANTHEREAL you can see the output in "process notebooks": https://github.com/microsoft/autogen/actions/runs/8922792168/job/24505683517?pr=2562#step:8:75

Error: agentchat_oai_assistant_function_call.ipynb: Title not found in notebook

You can fix that notebook and then it will be processed and converted into a webapge in CI.

To test it locally, in /website, you can run python process_notebooks.py render

[IANTHEREAL]

@IANTHEREAL you can see the output in "process notebooks": https://github.com/microsoft/autogen/actions/runs/8922792168/job/24505683517?pr=2562#step:8:75

Error: agentchat_oai_assistant_function_call.ipynb: Title not found in notebook

You can fix that notebook and then it will be processed and converted into a webapge in CI.

To test it locally, in /website, you can run python process_notebooks.py render

Thanks, error had been fixed