docs: add FAQ to crate-level docs

Author: alltheseas

crates/egui/src/lib.rs

@@ -315,6 +315,20 @@
 //!
 //! See [`Context::request_discard`] and [`Options::max_passes`] for more.
 //!
+//! ## FAQ
+//!
+//! **How do I store state between frames?**  
+//! Keep your own `struct` with the values you mutate (e.g. selected enum variants, slider numbers) and update it inside `App::update`. egui only borrows your data temporarily each frame - it never stores it for you.
+//!
+//! **Why does my layout jitter the first frame?**  
+//! Some widgets (e.g. [`Grid`]) need a frame of measurements before they can allocate perfectly. Call [`Context::request_discard`] or keep the widget alive so the second frame reuses the cached layout.
+//!
+//! **How do I make widgets span the full width?**  
+//! Use helpers like [`Ui::vertical_centered_justified`] or wrap content with [`Ui::with_layout`] and [`Layout::top_down`]/[`Layout::left_to_right`] to control justification explicitly.
+//!
+//! **Where can I see more complete examples?**  
+//! The live demo at <https://www.egui.rs/#demo> mirrors the code in `crates/egui_demo_app` and `crates/egui_demo_lib`. Each window links back to the relevant source so you can inspect a working reference implementation.
+//!
 //! # Misc
 //!
 //! ## How widgets works