The landing page contains the previews for each layout.

The links for opening the sidebars have been declared for shallow
routing because there are no major changes in other contents of the
pages.

This implements the router based on Next.js for left-sidebar-with-menu
layout.

In addition, the other layouts have been genericized for wider framework
support, although the underlying API is still based on Next.js
components.

The styling for mobile has been implemented.

Add files from create-next-app