Revert examples change + add doc.

examples/component/menu_in_frame_horizontal.cpp

@@ -5,16 +5,11 @@
 #include <string>  // for string, basic_string, operator+, to_string
 #include <vector>  // for vector
 
-#ifndef FTXUI_BUILD_MODULES
 #include "ftxui/component/captured_mouse.hpp"      // for ftxui
 #include "ftxui/component/component.hpp"           // for Radiobox, Renderer
 #include "ftxui/component/component_base.hpp"      // for ComponentBase
 #include "ftxui/component/screen_interactive.hpp"  // for ScreenInteractive
 #include "ftxui/dom/elements.hpp"  // for operator|, Element, size, border, frame, HEIGHT, LESS_THAN
-#else
-import ftxui.component;
-import ftxui.dom;
-#endif
 
 using namespace ftxui;