docs app style updates

[Narretz]

This updates the docs app to the new header style found on angularjs.org.

I've also taken the liberty to update the menu entries on the doc site - pending approval I'll update the org site, too (would fix #14351)

And fixes #14963

Notable differences between org / docs:

• search icon
• font family / weight (should we update?)
• no drop shadow on navbar because there's a secondary bar on docs

Notable differences between docs old / new:

• mobile: the top header + search are fixed, sub header is relative (previously all relative)
• mobile search close icon has moved over the search bar, because I had some problems giving it it's previous position while keeping the search results scrollable (oh my)

Implementation notes:

• angular-topnav.css is included as is from org site
• several overrides necessary because org uses bootstrap 2 / docs uses bs 3 (oh my)
• org site uses one breakpoint at 800px, docs uses one at 768 and one at 992. I tested it and it doesn't affect the common screen sizes

Future work:

• clean up the css and maybe implement sass for the docs app

Screens:

[gkalpak]

Didn't serve it locally, but the code and the screenshots look very good 😃

[gkalpak]

👍 😃 (The #!/ could be dropped: https://twitter.com/angular)

[Narretz]

Tests have been fixed

[IgorMinar]

lgtm. please make the same menu change as with the angularjs.org menu (faq/tutorial swap).

thanks!