docs: add Architecture Overview Guide (Closes #137)

[zeroknowledge0x]

Summary

Adds a single high-level architecture guide at the repo root that gives
new contributors a unified mental model of NotifyChain before they dig
into the per-component docs.

Issue: #137 — Documentation > Create Architecture Overview Guide.

What's in this guide

• Three-layer system overview — on-chain contracts, listener service, dashboard
• Contract catalog (TaskBounty + AutoShare) with on-chain/off-chain boundary contract
• Listener module map + scheduler subsystem with the atomic UPDATE ... WHERE status = 'PENDING' rationale
• End-to-end data flow — from contract invocation through event emission, deduplication, dispatch, and dashboard render
• Project structure tree and a first-day contributor checklist
• Cross-references to every subsystem doc, runbook, and reference spec
• Glossary of recurring terms (Soroban, cursor, dispatcher, DCO, GrantFox)

482 lines, well under the size that would warrant splitting.

Acceptance criteria mapping

Criterion	Where it's covered
New contributors understand project structure	§2 (system at a glance), §7 (project structure), §8 (where to start)
Documentation is easy to follow	§1–§11 with consistent heading hierarchy; TL;DR diagram in §2
Diagrams are included	§2, §4.1, §4.3, §6 (4 ASCII diagrams)
Document system components	§3 (contracts), §4 (listener), §5 (dashboard)
Explain data flow	§6 (end-to-end trace with 8 numbered steps)
Link related resources	§9 (related resources, grouped by purpose)

Files changed

• ARCHITECTURE_OVERVIEW.md — new (482 lines)
• README.md — link from the existing Architecture Overview section + entry in the project structure tree

Test plan

This is a documentation-only PR. No code changes.

• All internal links use repo-relative paths verified by grep
• Heading hierarchy is consistent (h1 → h2 → h3)
• No broken relative links (grep -oP '\]\([^)]+\)' returns 0 matches)
• Reviewed against existing Documents/Task Bounty/ARCHITECTURE.md and listener/ARCHITECTURE-DIAGRAM.md to avoid contradictions

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Closes #137

Signed-off-by: zeroknowledge0x