Add nice detail pages with downstream version availability information

[tianon]

@paultag talked about this in #hy recently, wanting to do some fun scraping and stuff to make sure it's always up-to-date

[tianon]

Here's something I wrote to gather version information for "hylang" on the Docker Hub:

(import requests)

(for [tag (.json (requests.get "https://index.docker.io/v1/repositories/hylang/tags"))]
    (print (get tag "name")))

(please don't judge me too hard, this is admittedly one of the first Hy scripts I've ever written, which is why I'm pasting it here instead of in a PR 💃)