Add rating board contract example

[alexgduarte]

Summary

Adds an original simple-tier rating-board CosmWasm example.

The contract lets users create named items and lets each address rate each item once, with support for updating an existing rating. Aggregate totals are stored on the item, so average-rating queries do not scan every individual rating.

What is included

• rating-board contract with instantiate, execute, query, state, and error modules
• README with example instantiate/execute/query messages
• Root README link under the introduction examples
• Seven unit tests covering config, item creation, aggregate rating updates, invalid input, rating queries, item pagination, and owner-only config updates

Verification

• cargo fmt
• cargo test
• git diff --check

Note: Cargo prints the same .cargo/config deprecation warning used by existing examples, but the tests pass.

Bounty / authorship

Related to #11 simple contract tier.

This is an original contract example authored for this PR. Contact for payout coordination: GitHub @alexgduarte; stablecoin payout details can be provided privately if accepted.