docs(usage): update usage page with import examples

[andrewhl]

Closes #1330.

[levithomason]

I think I'd rather link to some of the existing examples to show how to use components. There are several advantages to doing this. The examples have automated tests to ensure they work, the code is editable, they are actively managed, and they are already done :) I also like the idea of directing the user on through the docs to learn more after setup.

The examples have another cool feature. If you link directly to an example's anchor link, the page will load with the code editor open and show the rendered HTML:

http://react.semantic-ui.com/elements/button#button

Given this, let's update this section to inform the user that every example includes fully functional and editable code snippets. Then, let's link to 3 or 4 in a , using the basic button I used above as the first link.

[codecov-io]

Codecov Report

Merging #1336 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1336   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files         140      140           
  Lines        2402     2402           
=======================================
  Hits         2396     2396           
  Misses          6        6

Impacted Files	Coverage Δ
src/modules/Popup/Popup.js	100% <ø> (ø)	✅
src/modules/Popup/PopupHeader.js	100% <ø> (ø)	✅
src/modules/Popup/PopupContent.js	100% <ø> (ø)	✅

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[andrewhl]

@levithomason Okay, I've updated this PR with a simple UL containing some links to example components. Please let me know if this is what you had in mind. Cheers

[levithomason]

Thanks!