Add example of graceful degradation

@prushforth

it's worth making an example, if we can, of the built-in that falls back to an image map in Safari.

Originally posted by @prushforth in #39 (comment)

There's an example in index-map-area.html. We can copy & paste relevant parts and add to high-level-api.md#graceful-degradation-and-progressive-enhancement. I've tested it in other browsers that don't support built-ins such as IE 10, but it'd be great if someone can confirm the fallback works correctly in Safari as well.

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