docs: Add AI contribution note and style guideline for type annotation of return values

[tonyandrewmeyer]

Two small updates to our contribution and style guidelines:

• A statement on use of AI tools, summarising our discussion in Sweden. Basically: leave the Copilot reviews to us, but you're welcome to submit PRs that were (co-)written with AI, as long as you're a human taking responsibility for it.
• An update to our style guide recording our decision on when a type annotation for the return value of a function or method should be provided.