feat: Enhance backend architecture documentation and conventions - Updated code style asset links in Profiles and Environment Separation documentation. - Added conventions and endpoints documentation for the backend component. - Created overview and patterns documentation for the backend component. - Introduced stubs documentation for both backend and legacy components. - Migrated legacy component documentation, including conventions, endpoints, and patterns. - Removed outdated INDEX.md files from backend structure. - Added detailed documentation for audit tables and correlation ID connection provider. - Introduced District Average Parity Fixtures for backend testing.