intro_section_with_dropdown_navigation done in react.js, first attempt

This is my initial attempt. There is more work to do yet.

I am submitting so I can peek at some other solutions to figure out how to re-use the drop-down menus in desktop and in mobile.

Also there is a problem with the hero image. No matter how small I make it, I get horizontal scroll bars. If I commend out the image, the scroll bars disappear.