Walk through USAGE.md and make any necessary changes to ensure the path works

[sethladd]

Let's double check the instructions in USAGE.md still work.

Let's also ensure the positioning of this doc is clear: this is a usage in the context of firebase AI logic and GenUI.

Consider reframing this as a "codelab", as it's less about using GenUI and more about how to get started. So maybe we rename the file to "getting_started_with_genui_and_firebase_ai_logic.md" ?

[sethladd]

cc @polina-c

[sethladd]

Bumping to P1 as we want our USAGE.md to be correct, as people are now following it