`egui_demo_app/lib`: add "About egui" window, and improve mobile layout
This makes the app responsive, removing the side bars on mobile and turning them into drop-down menus instead.
* Remove integration name (it is always eframe)
* Remove egui_web crate
* Move egui_web/CHANGELOG.md into eframe/CHANGELOG.md
* Remove all mentions of egui_web
* Remove epi crate and absorb into eframe
* egui_glow: only use puffin on native
* Remove WASM doc from CI (we don't generate it anyways!)
* Remove eframe::epi and improve eframe docs