docs: add end-to-end developer journey documentation

[CyberXpert607]

Summary

Implements #40: End-to-end developer journey documentation for AnchorKit.

Changes Made

1. Developer Journey (docs/DEVELOPER_JOURNEY.md)

• ✅ Overview of AnchorKit and monorepo structure
• ✅ Account diagnostics flow
• ✅ Payment readiness flow
• ✅ Anchor lifecycle flow
• ✅ Escrow milestone flow
• ✅ Mermaid flow diagram
• ✅ Code examples with links to actual files
• ✅ UI references
• ✅ Test references
• ✅ Roadmap notes
• ✅ Cross-link to capability exports

2. Docs Navigation (apps/web/app/docs/page.tsx)

• ✅ Added developer journey to docs index
• ✅ Featured as Step 1 in "Where to read next"

3. README.md

• ✅ Fixed broken merge conflict markers
• ✅ Reformatted with proper markdown
• ✅ Added link to developer journey

4. Merge Conflict Resolution

• ✅ Resolved conflicts in README.md after syncing with upstream/main

Related Issue

Closes #40

Checklist

• End-to-end journey documented
• Account diagnostics flow included
• Payment readiness flow included
• Anchor lifecycle flow included
• Escrow milestone flow included
• Examples and tests linked
• README links to the journey
• Docs navigation includes the journey