docs: add JavaScript SDK examples to contract API

[Oluwasegun6921]

Summary

Adds copy-paste-ready JavaScript examples to docs/CONTRACT_API.md for all public Accord contract functions.

What changed

• Added a shared @stellar/stellar-sdk setup section.
• Added state-changing examples showing simulation, transaction assembly, signing, and submission.
• Added read-only examples using simulateTransaction and scValToNative.
• Used the appropriate Soroban value encodings for addresses, amounts, and u64 proposal IDs.
• Kept the changes limited to docs/CONTRACT_API.md.

Testing

• Verified the JavaScript examples for syntax errors.
• Confirmed no source code or unrelated files were modified.

Closes #134

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