Add hash notary contract example

[nunezve]

Summary

Adds a new original hash-notary CosmWasm example contract.

• registers normalized SHA-256 document/artifact digests with a label and optional memo
• supports owner-only memo updates and revocation while keeping the original proof queryable
• adds digest/id/list/state queries, schema generation, README documentation, and a root README link
• includes unit tests for registration, duplicate prevention, owner permissions, validation, and pagination

Bounty / Originality

Bounty issue: #11

This contract was written from scratch for this PR. It is not copied from another contract or existing example.

Contact for review/payout coordination: GitHub @nunezve. Stablecoin payout details can be provided privately if accepted.

Verification

• cargo fmt --manifest-path hash-notary/Cargo.toml -- --check
• cargo test --manifest-path hash-notary/Cargo.toml (5 passed)
• git diff --check