Add new component and Fix docs

[gjssss]

Brief description

Hey, I edit my code again, which I PR before. 279

• I add a new dropdown component and fix the bug rendering in mobile device. (pop a dropdown window when click it)
• Fix the navbar docs. (remove fixed in html and add a lang="en" attribute in html tag)

Problem:
When I click it twice in mobile simulator in browser, the color of text is the hover color but not primary color like the picture below.

Developer Certificate of Origin

• I certify that these changes according to the Developer Certificate of Origin 1.1 as described at https://developercertificate.org/.

Sample pictures

Further details

I will fix the bugs and add some animation. I have no experience of pull a request, so if there have something wrong please give me a notice. It is your warm help and patience that enable me learn how to use GitHub. ^ ^