Publish the AppConfig JSON schema

There’s already a related topic, and more than a few people are either interested in it or have given up.

For VSCode, just update the URL and everything should be fine: https://github.com/tamasfe/taplo/blob/42bce0c6a753a394ca40263660a889165c2762e5/site/site/public/schema_index.json#L12-L20