Site tidyup for ui-grid.info

Filled in missing content on page, added a more complex example,
shifted link from API to tutorials.

I considered including more content in line with the old ng-grid page, but most of this content has now moved to the tutorial or API sections, and doesn't belong on the front page.  I also looked at the data-tables.net, and there's really the same content (they have a forum and some other bits, but really no more information than here).

So, I'm going with this for now until I think of something better.