Responsive Web Features

1. Layout of grid:
   The boxes of content will become 2*2 when the scale of webpage increases. It will become 1*4 when the scale of webpage decreases.
   
2. Layout of webpage:
   Navbar-toggler will hide the list of button when the width of page is small. But it will automatically show the list of button when the width of page is large enough.
3. Using the container idea:
   Ensure the size of content will not too thin or wide.
4. Button:
   To hide/ show the list of the special link.