Add storage-tier documentation for validators

[Manta-Byte]

Summary

Documents every storage key used in the validators contract, organized by Soroban storage tier (Instance / Persistent / Temporary), with rationale for each tier classification.

Changes

• Added contracts/validators/docs/storage.md: a full tier reference covering all storage keys used by the contract (Admin, StorageVersion, AuthorisedUpgrade — all Instance tier), including a tier-overview table, per-key access metadata, TTL management strategy, and explicit rationale for why the Persistent and Temporary tiers are unused here.
• Enhanced inline /// rustdoc on the StorageKey enum in migrate.rs with per-variant tier rationale matching the docs.
• Added 10 new unit tests (20 → 30 total) covering tier segregation (confirming reads/writes stay on the Instance tier), TTL bump-path behavior, an upgrade-overwrite case, and an enum exhaustiveness guard.

Security checklist

• require_auth on all state-changing entrypoints
• No unwrap() in production paths (only unwrap_or/unwrap_or_else with safe defaults)
• Overflow-safe arithmetic (checked_add)
• Rustdoc present on all storage-related public items

Closes #719

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