Move some of our documentation to docs.rs (#3478)

* Move platform-specific documentation to `winit::platform` module

* Document cargo features in crate docs

* Move version requirements to crate-level docs

src/platform/x11.rs

@@ -1,3 +1,4 @@
+//! # X11
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};