Create a JSON Schema for every new version

[tomootes]

Currently we have the JSON-Schema at schemastore.org. For many purposes, like the publiccode.yml-editors that exist, it would be handy if we could use the JSON Schema of a specific version. Currently we can only link to this current version: https://github.com/SchemaStore/schemastore/blob/master/src/schemas/json/publiccode.json. It's not even pinned so i expect it to change when a new version gets rolled uit.

I would like to create a new JSON Schema for every new version. We can use this to validate publiccode.yml files and approach it as the single source of truth.

Imo this also removes the need for parser projects like https://github.com/italia/publiccode-parser-go/issues. It could simply be replaced by a generic tool which works with JSON Schema's. This eliminates a lot of work.