[Docs] High-level doc for communicating our supported LLM models

[LyalinDotCom]

Is your report related to a problem? Please describe.
It's hard to find which models we support today, its available in our prompting docs but is hard to discover. Let's makes this a lot easier to discover.

Some specifics:

• Today this information is here: https://firebase.google.com/docs/genkit/models
• I think it should be a doc by itself under the top TOC near "introduction" along with some high-level overview in introduction doc itself
• Ideally we'd continue to list both supported models, and those that are community projects