Move examples

[jakevdp]

Currently example code lives in a somewhat confusing place. I think it would be better to move examples to a top-level location before the vega-lite 3 migration, so that it will be easier to test all of them without moving files.

[jakevdp]

This is mostly easy, except for making certain the doc build correctly finds the new example location.

There might also be some contributor docs floating around that would need to be updated.