Suggestion to link to sponsors images

[siaw23]

When you visit https://rubyonrails.org/world and scroll down, you'll see a list of sponsors. I think it'd be a nice touch to link the images somehow to the sponsors' sites. This will serve a few purposes:

1. An easy way to visit the sponsor's site
2. A way to find out what the sponsor does, for example. I don't know "The Agency" and can't find them on Google as there are quite a few "agencies".

@AmandaPerino

[AmandaPerino]

Done with #171

[AmandaPerino]

Thank you @siaw23 !