pdoom-data README overhaul: schema, provenance, contribution rules

[PipFoweraker]

Tasks

• Document core datasets and how they're used (game vs website)
• Add schema notes (fields, required columns, formats)
• Add provenance rules: sources, citations, and review standards
• Add contributor workflow: how to propose events / corrections

Acceptance Criteria

• A donor can understand why this repo is a reusable public good in 3 minutes

Part of Grant Readiness Epic (pdoom1#479)