Update README & Quickstart to feature `gr.Server`

I think we should rewrite our docs to emphasize you can use Gradio in 3 levels of complexity: - `gr.Interface` - simplest, no UI flexibility - `gr.Blocks` - medium, UI written in Python - `gr.Server` - most custom, requires (vibe)-coding your own frontend in JS And explain why you'd use it today (comparing to other frameworks / vibe-coding your own web app)