Docs: document Downloads API #2750

Summary

It seems like the existence of the Downloads API is not documented, other than by its presence in the source code.

I think it'd be a good idea to document its existence somewhere on the site itself, probably under an "API" link in the index on the bottom of each page? Since I think that's what most people would expect, from how other sites do it, and there have been issue(s) in the past with APIs being forgotten about and stopping working.