code: public HandlerBuilder<ContextHandler> createRootContextHandler(String subPath) {
          ContextHandler contextHandler = new ContextHandler();
          HandlerBuilder<ContextHandler> e = new HandlerBuilder<>(contextHandler);
          String usePath = contextPath + subPath;
          setPath(contextHandler, usePath);
          handlers.add(e);
          return e;
      }
docs: Creates a HandlerBuilder that is mounted on top of the root path of this builder
  
  @param subPath The sub-path that will be appended, starting with a slash, or just an empty string for no sub-path
  @return A handler builder. This can not be used for servlets, see #createRootServletContextHandler