Gallery updates

This PR adds an entry to the example gallery, and fixes it up to be a bit more consistent.

I tried to build this locally, but I got a ton of sphinx warnings of all sorts, even before this PR.  @keflavich, do you see the same thing, or do the docs build for you without complaint?

Note that I also elminated the (old) Exmaple 3, because it didn't work.  Let me know if it is actually supposed to work, @keflavich, and if so, how it might be fixed.