Navbar links and copy editing

On most websites, the top leftmost header image is a link to the main "index.html" page on that particular website, and not a different URL. These edits reflect that convention.

Because we also want to link to the main GeoTrellis docs site (http://geotrellis.github.io/) somewhere, I added an additional "GeoTrellis Docs" link on the far right by the other navbar links.

Also edited an example question for clarity.