docs(architecture): add request lifecycle documentation for Arcadia API
docs(architecture): add execution, transactions, and domain events documentation for Arcadia API
docs(architecture): add guide for creating a new use case in Arcadia API
docs(exceptions): summarize exception handling and HTTP status codes in Arcadia API
docs(tests): outline testing strategy and conventions for Arcadia API
style(openapi): correct description formatting in OpenAPI documentation
chore: remove outdated exceptions documentation
test: remove obsolete tests documentation
