DOC: add TODO to migrate raw raises to custom RocketPy exceptions

Addresses the review note in exceptions.py: reminds contributors to
progressively adopt the custom RocketPyError subclasses across the codebase
(and add new exception types here) instead of raising bare ValueError/TypeError.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Gui-FernandesBR

rocketpy/exceptions.py

@@ -1,5 +1,10 @@
 """Custom exceptions and warnings for RocketPy."""
 
+# TODO: progressively adopt these custom exceptions across the codebase.
+# Many modules still ``raise ValueError``/``TypeError`` directly; migrate those
+# to the appropriate ``RocketPyError`` subclass (adding new exception types here
+# as needed) so users can reliably catch ``RocketPyError`` and its subclasses.
+
 
 class RocketPyError(Exception):
     """Base class for all RocketPy exceptions."""