add(element): interface for injecting user behavior to containers

Some additional refactoring and documentation updates have also been
applied.

src/zterm.zig

@@ -5,6 +5,9 @@ const size = @import("size.zig");
 
 // public exports
 pub const App = @import("app.zig").App;
+// App also exports further types once initialized with the user events at compile time:
+// `App.Container`
+// `App.Element`
 pub const Renderer = @import("render.zig");
 
 // Container Configurations