Update docs: Add information about the Google provider

[hd24-jspoerer]

Adjust the docs to reflect the below change.

Paraphrased from @evalstate's comment from the Google provider PR:

https://github.com/evalstate/fast-agent-docs -> T rules over the API keys have changed (does it look for both VERTEX and GOOGLE_API_KEY). Also, the old provider is now accessible as "googleoai." and takes api keys in the same way.

See also the Google provider PR: #134

I'll try to do this within the next two weeks. If anybody gets to this earlier, please feel free to do it. Thanks.