docs: add observability, creating modules guide, and unified migration plan

- Add observability framework documentation (health checks, metrics, Sentry)
- Add developer guide for creating modules
- Add comprehensive module migration plan with Celery task integration
- Update architecture overview with module system and observability sections
- Update module-system.md with links to new docs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mkdocs.yml

@@ -41,6 +41,7 @@ nav:
       - Middleware Stack: architecture/middleware.md
       - Module System: architecture/module-system.md
       - Menu Management: architecture/menu-management.md
+      - Observability: architecture/observability.md
       - Request Flow: architecture/request-flow.md
       - Authentication & RBAC: architecture/auth-rbac.md
       - Frontend Structure: architecture/frontend-structure.md
@@ -115,6 +116,7 @@ nav:
     # --- Development ---
     - Development:
       - Contributing Guide: development/contributing.md
+      - Creating Modules: development/creating-modules.md
       - Code Quality: development/code-quality.md
       - Architecture Rules: development/architecture-rules.md
       - Security Rules: development/security-rules.md
@@ -157,6 +159,7 @@ nav:
       - Vendor Operations Expansion: development/migration/vendor-operations-expansion.md
 
     - Implementation Plans:
+      - Module Migration Plan: proposals/module-migration-plan.md
       - Admin Inventory Management: implementation/inventory-admin-migration.md
       - Admin Notification System: implementation/admin-notification-system.md
       - Letzshop Order Import: implementation/letzshop-order-import-improvements.md