docs(staking): document stake position storage model

[bamiebot-maker]

Summary

Closes #538
Documents the staking position storage model for contributors and integrators.

Changes

• Documents StakePosition fields: stake_id, amount, and unlock_ledger
• Explains sequential stake ID assignment per wallet and creator, starting at 1
• Describes the composite storage-key structure for positions
• Notes that wallets may hold multiple concurrent stake positions for the same creator
• Links the new documentation from the README and storage-key invariants guide

Validation

• Confirmed the staged documentation diff has no whitespace errors with git diff --check