Added pre-commit hooks (#498)

[COUR4G3]

Added pre-commit hooks for checking and updating news in projects using pre-commit (#498)

• added towncrier-check to just check newly commited news fragments
• added towncrier-update to actually update the news
• added documentation section for pre-commit

Description

Added a pre-commit hooks for checking and updating news in projects using pre-commit, as suggested in my issue #498.

Checklist

• Make sure changes are covered by existing or new tests.
• For at least one Python version, make sure local test run is green.
• Create a file in src/towncrier/newsfragments/. Describe your
  change and include important information. Your change will be included in the public release notes.
• Make sure all GitHub Actions checks are green (they are automatically checking all of the above).
• Ensure docs/tutorial.rst is still up-to-date.
• If you add new CLI arguments (or change the meaning of existing ones), make sure docs/cli.rst reflects those changes.
• If you add new configuration options (or change the meaning of existing ones), make sure docs/configuration.rst reflects those changes.

[hynek]

Thanks! We use semantic linefeeds for docs but until #500 is merged it's not documented so I did it inline.