Improve TestCase note example

[jeet-dekivadia]

Purpose

Improve the marbles.core.TestCase docstring example so it demonstrates why a note is useful when external data changes unexpectedly.

Fixes #117.

Approach

Add a focused external-catalog example for the TestCase docstring while leaving the quickstart tutorial fixtures unchanged. The note explains the expected threshold and what a lower count may indicate.

Tests for New Behavior

Documentation-only change.

• git diff --check
• python3 -m py_compile docs/examples/catalog_data.py marbles/core/marbles/core/marbles.py
• confirmed the new Sphinx literalinclude target exists

Checklist

• Signed and returned a copy of the CLA
• Added tests for any new features or behaviors
• Ran nox -s flake8 to make sure code style is consistent
• Built and reviewed the docs
• Added a note to the changelog

Learning

The existing quickstart example is useful for showing failure output, but a separate docstring example makes the value of author-provided context clearer without changing generated quickstart fixtures.

[jeet-dekivadia]

I checked the failing checks here as well. The Python package matrix is failing in the existing marbles/core/tests/test_setuptools.py path because current Setuptools has removed support for setuptools.command.test; the failures happen while the example packages invoke the legacy test command. The Read the Docs link check also appears to be an integration/setup failure rather than a failure in this diff.

This PR still only changes the TestCase note example plus its changelog entry and literalinclude example, so I do not see a patch-side code change to make for those CI failures.