Categorize UIExamples a little bit

Summary:
Ref M1476. I'm planning to add some PHUIX examples, but sort out the existing examples a little first. I added some categories:

• Catalogs: these are what I look at most often (emoji, icons, colors).
• Single use: elements with only one use (badges, feed stories, hovercards, setup issues).
• Technical: examples which are really just "test this thing in the browser" (avatars, gestures, notifications, remarkup).
• Other: evrything else, mostly general-purpose multi-use components.

Test Plan:
(See left nav.)

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D18050