Add navigation bar

[djhoese]

I think this page was originally designed as a single page of pytroll related information. However, we've now grown the site to include other pages (logos, workshops, etc). These pages are sometimes hard to find because there is no navigation bar on the side or top of the page to these other pages; only links at the bottom.

I propose we find a new theme or update the existing one to include a navigation bar somewhere. Might give us a chance to remove some of the extra text at the top of the page that I think we didn't want (View on GitHub, the double Pytroll page title, etc).