Add guestbook contract example

[alexgduarte]

Summary

Adds an original simple-tier guestbook CosmWasm example.

The contract lets any address sign a guestbook with one current entry, update that entry by signing again, remove its own entry, and query entries with pagination. The owner can update the title/message limit, transfer ownership, and remove any entry.

What is included

• guestbook contract with instantiate, execute, query, state, and error modules
• README with example instantiate/execute/query messages
• Root README link under the introduction examples
• Six unit tests covering config, signing/updating, validation, deletion rules, pagination, and ownership transfer

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.