Publish `schema/flagsmith.json` to JSON Schema Store

- Add `schema/flagsmith.json` to https://github.com/SchemaStore/schemastore. Popular IDEs will start picking up `flagsmith.json` schema automagically.
- Remove explicit schema reference from `flagsmith.json` generation.

Start with schema/flagsmith.json and trace how it is generated, noting where the explicit schema reference is added. Review SchemaStore’s contribution requirements, then verify that the schema is accepted there and that generated flagsmith.json no longer contains the explicit reference.

索引モデルが issue の本文から書いたものです。