Add developer docs: overview, architecture, and demo guide

[cargopete]

Summary

• Adds docs/overview.md — what SSDS is, where it fits in The Graph ecosystem, payment flow summary, and current MVP state
• Adds docs/architecture.md — component reference, network topology diagram, ports table, key design decisions, and repo layout
• Adds docs/demo-guide.md — step-by-step guide for running the full local stack and streaming data through the payment loop (both reflex and manual modes)
• Adds docs/SUMMARY.md — GitBook-compatible navigation index linking all docs

Test plan

• Verify all links in SUMMARY.md resolve correctly
• Follow the demo guide end-to-end and confirm accuracy