🐛 Avoid duplicate Towncrier markdown title headings

[acabarbaye]

Summary

• avoid prepending another Markdown heading marker when Towncrier title_format already defines one
• keep the existing changelog post-processing workaround and add coverage for the temporary config path
• document the required Towncrier configuration in DEVELOPMENT.md

Testing

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 pytest -o addopts='' tests/versioning/test_generate_news.py