Refactor to follow convention of saving API_KEY in `.env` file

[eyefodder]

Putting the API_KEY in plain text in the 00_Tutorial... notebook feels like an easy way for folks to accidentally check an API key into source control.

I updated it to follow the same pattern used in anthropic_api_fundamentals. For the choice of model itself, I pulled this up to the individual notebook as it felt more readable, but could see an argument for putting that in .env too.