docs: add local sandbox quickstart

[Epic-Drytu]

Description

Adds a Local Mock Sandbox section to the Quickstart guide to help developers run FlowFi locally using mock services and seeded data.

Changes Made

• Documented prerequisites and startup instructions.
• Explained the local infrastructure and service ports.
• Added application endpoints and shutdown instructions.
• Clarified the differences between Local Sandbox and Stellar Testnet.
• Documented mock RPC limitations.
• Added troubleshooting guidance and database safety notes.
• Preserved the existing Testnet quickstart.

Type of Change

• Documentation update

Testing

• Docusaurus documentation build
• git diff --check
• Manual browser preview

Related Issue

Closes #1664

[Epic-Drytu]

Hi maintainers! Thanks for coordinating the related work.

I noticed PR #1674 also includes the Local Sandbox documentation alongside the frontend component tests. My PR #1673 focuses specifically on the Quickstart documentation for issue #1664, and its automated checks are passing.

I’m happy to coordinate with the other contributor to avoid duplicate documentation. Please let me know whether you’d prefer these contributions to remain separate or be handled together.

Thanks!