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/windows.rs

@@ -1,3 +1,7 @@
+//! # Windows
+//!
+//! The supported OS version is Windows 7 or higher, though Windows 10 is
+//! tested regularly.
 use std::{ffi::c_void, path::Path};
 
 use crate::{