docs: Write a CLI manual testing guide for contributors

[AlAfiz]

Description

When new backend or smart contract developers join, they struggle to figure out how to interact with the contract locally.
We need a step-by-step markdown guide showing exactly how to use the stellar contract invoke CLI commands.
This guide should cover how to initialize the pool and execute a basic swap using dummy CLI identities.
Having this ready will drastically speed up onboarding for Wave 3.

Requirements

• Create a docs/CLI_TESTING_GUIDE.md file.
• Write out the exact bash command required to deploy the contract to a local sandbox.
• Provide the exact stellar contract invoke command to call the initialize function.
• Provide a sample command to call the swap function with dummy integer arguments.