Improve onboarding: minimal “Hello Vello” example missing

Why this matters Vello has: powerful architecture many crates multiple examples (winit, wasm, android) But no single “5-minute success path” Suggested issue title Add minimal quick-start example for first-time users Core problem New users must jump through: examples directory cargo flags platform notes (WASM / Android / winit) Suggestion Add a section that includes: 1 install command 1 render command expected output (image/window)