Add branding to hover

[ericholscher]

It would be great if there was an optional way to include a small brand for the extension on the hovers, so folks know more about how it is being generated. I'm thinking of something similar to what we're doing on the search extension, where there's a small brand for the extension. For search we put a larger RTD brand on there since it's more of a page-level UX element, but I think on the hovers it could just be a small generated by sphinx-hoverxref [docs] or something.