Start a "principles" and/or "best practices" guide

Values could be:

- Use [sentence case](https://apastyle.apa.org/style-grammar-guidelines/capitalization/sentence-case) for user-facing output
- Be as specific as possible (and no more) when raising an error (e.g. where appropriate, prefer `ValueError` to `Exception`)

(I have more principles in a previous iteration of democritus)

Examples/inspirations:
- https://github.com/best-doctor/guides/blob/master/guides/en/python_styleguide.md

No repository file, test, or entry point is identified. Start by reviewing the proposed values, the previous democritus iteration, and the linked Python style guide; done means adding a principles or best-practices guide with an agreed scope and examples.

Written by the indexing model from the issue text.