Update Cookbook introduction

[paytonison]

Hi,

I was thumbing through the introduction article for the Cookbook and saw that it's really old and not that helpful. Since GPT-OSS was released, and the references to GPT-3 and 4 are irrelevant.

I think it's important that GPT-OSS is mentioned in the introduction because it gives people exploring language models a concrete way to test ideas and play with models without using the API if they don't feel like going through the process.

I've been working on my own version of the introduction with PR #2121 if that makes a difference.