Document local Stellar sandbox setup

[agnesadoga-creator]

Closes #84

Summary

Adds a walkthrough for setting up a local Stellar/Soroban sandbox for this repo.

Details

Adds docs/LOCAL_SANDBOX_SETUP.md, a step-by-step guide grounded in this repo's actual tooling:

• Starting the standalone network already defined in docker-compose.yml (stellar/quickstart image)
• Configuring .env.local.example values
• Generating/funding a local Soroban CLI identity
• Building WASM per rust-toolchain.toml + the soroban-sdk 20.0.0 pin in Cargo.toml
• Deploying each contract (invoice, treasury, compliance) to the sandbox via the Soroban CLI, since no deploy_local.sh script currently exists
• Recording deployed IDs via the existing scripts/export_deployed_addresses.sh
• Invoking a contract as a sanity check, plus a troubleshooting section

Also adds one link from README.md's Local development section to the new doc, matching the existing pattern used for docs/MAINNET_DEPLOYMENT.md.

No contract source, Cargo.toml, or CI config changed.