Improve event-db environment specific data documentation

- [ ] Change `stage` to `environment` (in the files introduced here: https://github.com/input-output-hk/catalyst-core/pull/486)
- [ ] Improve the environment specific data [README](https://github.com/input-output-hk/catalyst-core/blob/ddca820715d2302c62c04abbb217f604970b4639/src/event-db/stage_data/Readme.md) file