chore: set up + document commitizen-cli for authoring clear commit messages

[hydrosquall]

Motivation

• We introduced chore: pre-commit hooks for code style and commit message quality #66 to help improve changelog generation, but developers may not remember the recommended prefixes. Using a CLI can help. (If people prefer not to use it, they can write commit messages normally).
• Quality of Life updates to repo #47 (comment)

Changes

• Followed the installation guide: https://github.com/commitizen/cz-cli
• Put project config in a .czrc file instead of adding mysterious new properties to package.json
• Documented usage in CONTRIBUTING.md + in a project badge in the top-level README

Testing

• Tested yarn commit locally, and went through the interactive prompts to write both commit messages in this PR.

[captainsafia]

Looks good!

[github-actions]

🚀 PR was released in v8.2.12 🚀