Add a gallery of results to pyhf project page

[matthewfeickert]

Some helpful feedback received from @davidlange6 on how to improve the pyhf project page was

An example addition might be a gallery of results (basically graphics from the "use in publications" list).

There are already plenty of results that exist across the internet that can be shown, including results from SModelS, so this would be an easy addition.

A note to self: The pyhf page is already pretty list-y, so maybe try to "lead with awesome" and get some of the example plot up at the top.