docs: add __init__.py with docstring for openapi module

The openapi directory needs an __init__.py to be recognized as a Python package
by the documentation build system (mkdocstrings). Added a descriptive docstring
to document the module's purpose without exporting any symbols.

Author: yizzy

aws_lambda_powertools/event_handler/openapi/__init__.py

@@ -0,0 +1,5 @@
+"""OpenAPI validation and schema generation for AWS Lambda Powertools Event Handler.
+
+This module provides OpenAPI 3.0/3.1 schema generation, request/response validation,
+and Swagger UI integration for Lambda function URL, API Gateway, ALB, and VPC Lattice.
+"""