Revamp quick start

[capossele]

• Add overview section to README
• Update dependencies instructions
• Refactor BonsaiStarter test to include explicit mocks for both off-chain and on-chain callback requests
• Simplify the off-chain example
• Reorganize the deployment instruction to highlight the off-chain mode while still keeping the steps for the on-chain mode

Overall it's a small refactoring. I'm still not sure if this is enough for shifting the focus towards the off-chain requests.