Update website with link or port of cog intro

[cholmes]

There's a really nice explanation of COG's at https://medium.com/planet-stories/a-handy-introduction-to-cloud-optimized-geotiffs-1f2c9e716ec3

At the very least it'd be good to put a prominent link to it, as it's a great explanation. Ideally we'd even port a good bit of that info to the site itself, as it's a better explanation than anything on the site currently.