docs: add comprehensive README explaining repository structure and data schemas

[Copilot]

The README contained only a single line with no explanation of what the data files contain or how they're structured.

Changes

• Repository overview — folder tree annotated with purpose of each directory and script
• Per-file schema docs — purpose and JSON shape documented for all 24 data files across skyblock/, skyblock/networth/, and misc/; notable fields (e.g. optional stacking/ultimate/upgrade_stones on enchants, null sentinel in gemstone stat arrays) are called out inline
• Validation & contributing — documents how to run check.py and sets expectations for contributors

Example of the level of detail added for enchants.json:

{
  "<enchant_name_lowercase>": {
    "name": "<Display Name>",
    "id": "<internal_api_id>",
    "max_table": <max level from enchanting table>,
    "max": <absolute maximum level>,
    "stacking": true,        // present only for stacking enchants
    "ultimate": true,        // present only for ultimate enchants
    "upgrade_stones": {
      "<level>": "<ITEM_ID>" // present only where an upgrade stone is required
    }
  }
}

Original prompt

Explain this

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