Revert examples change + add doc.

examples/component/custom_loop.cpp

@@ -3,25 +3,16 @@
 // the LICENSE file.
 #include <stdlib.h>                   // for EXIT_SUCCESS
 #include <chrono>                     // for milliseconds
-
-#ifndef FTXUI_BUILD_MODULES
 #include <ftxui/component/event.hpp>  // for Event
 #include <ftxui/component/mouse.hpp>  // for ftxui
 #include <ftxui/dom/elements.hpp>  // for text, separator, Element, operator|, vbox, border
-#endif 
-
 #include <memory>                  // for allocator, shared_ptr
 #include <string>                  // for operator+, to_string
 #include <thread>                  // for sleep_for
 
-#ifndef FTXUI_BUILD_MODULES
 #include "ftxui/component/component.hpp"  // for CatchEvent, Renderer, operator|=
 #include "ftxui/component/loop.hpp"       // for Loop
 #include "ftxui/component/screen_interactive.hpp"  // for ScreenInteractive
-#else
-import ftxui.component;
-import ftxui.dom;
-#endif
 
 int main() {
   using namespace ftxui;