Add window plugin docs

docs/1_structure/2_engine/2_plugins/4_window.md

@@ -1,3 +1,10 @@
 # Window {#engine-plugins-window}
 
-TODO
+The \ref cubos::engine::plugins::window "window" plugin adds the \ref
+cubos::core::io::Window "core::io::Window" resource, and a startup system on
+the stage `"window"` which initializes it, by opening the window and creating a
+\ref cubos::core::gl::RenderDevice "core::gl::RenderDevice" for it.
+
+The window is created using the settings `window.width`, `window.height`,
+`window.title` and `window.vsync`. The default values are `800`, `600`,
+`"Cubos Game"` and `true`, respectively.