docs: add module-level docstring to logger module

The logger module lacked documentation explaining its purpose as the
default logging provider for FastAPI applications.

fastapi/logger.py

@@ -1,3 +1,10 @@
+"""
+Default logger for FastAPI.
+
+Provides a pre-configured logger instance named "fastapi" for internal
+logging throughout the framework.
+"""
+
 import logging
 
 logger = logging.getLogger("fastapi")