Revert examples change + add doc.

examples/dom/size.cpp

@@ -1,22 +1,14 @@
 // Copyright 2020 Arthur Sonzogni. All rights reserved.
 // Use of this source code is governed by the MIT license that can be found in
 // the LICENSE file.
-#ifndef FTXUI_BUILD_MODULES
 #include <ftxui/dom/elements.hpp>  // for operator|, text, Element, hcenter, Fit, hbox, size, window, Elements, bold, dim, EQUAL, WIDTH
 #include <ftxui/screen/screen.hpp>  // for Screen
-#endif
-
 #include <memory>                   // for allocator, shared_ptr
 #include <string>                   // for string, to_string
 #include <utility>                  // for move
 
-#ifndef FTXUI_BUILD_MODULES
 #include "ftxui/dom/node.hpp"      // for Render
 #include "ftxui/screen/color.hpp"  // for ftxui
-#else
-import ftxui.dom;
-import ftxui.screen;
-#endif
 
 int main() {
   using namespace ftxui;