Add some batch download links to the homepage

[daemontus]

In many cases, the user just wants to download all models in some easy to parse format. We provide this as part of the github releases, but I don't think there are links in the readme for this. We should add this to the readme as big, easy to find buttons.

Also, we have had several releases at this point, with gradually expanding batches of models. This part of the homepage could actually link to all the previous releases as well (although I am assuming this will be updated manually, since it changes roughly once per year anyway).