Add code understanding & testing point to Contributing Guidelines

[koibtw]

Type of Change

• [x] Documentation update

Description

We've seen some non-tested LLM-generated PRs in the past few days that are a pain to review. Adding this to the guidelines will save us some time not having to re-write bad, not working AI-generated code.

Checklist

• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no errors/warnings/merge conflicts.