docs: add module-level docstring to signals module

Documents the Flask signal system, explaining its role in providing
Blinker signals for key application events like request lifecycle,
template rendering, and application context changes.

src/flask/signals.py

@@ -1,3 +1,13 @@
+"""
+Application signals for Flask.
+
+Provides Blinker signals for key application events such as request
+lifecycle, template rendering, and application context changes.
+
+These signals allow decoupled components to react to framework events
+without modifying core code.
+"""
+
 from __future__ import annotations
 
 from blinker import Namespace