downloads: Add link to versions.json (#1101)

* downloads: Add link to versions.json

This file contains info on all published Julia versions (see JuliaLang/julia#33817 for more context).
It's currently not easy to find but may be useful for people who build Julia tooling.

Generation script: https://github.com/SaschaMann/Julia-versions.json/blob/main/build_json_map.jl (written by `@staticfloat`)
Repo that builds and deploys it: https://github.com/SaschaMann/Julia-versions.json

* Add link to schema

downloads/index.md

@@ -349,3 +349,9 @@ The platforms currently supported by Julia are listed below. They are divided in
 </ul>
 ~~~
 @@ @@
+
+
+
+## JSON release feed
+
+The info above is also available as a [JSON file](https://julialang-s3.julialang.org/bin/versions.json) ([schema](https://github.com/SaschaMann/Julia-versions.json/blob/main/schema.json)). The file is updated once every 24 hours.