Good First Issue: Improve documentation for writing custom rules

[ParzivalHack]

PySpector supports extensible rule definitions, but the documentation can be improved for new contributors.

Goal:

• Clarify how rules are structured
• Add a minimal step-by-step example
• Explain common pitfalls and best practices

This is a documentation-focused issue and does not require deep code knowledge :)

[Chidwan3578]

Done

[ParzivalHack]

@Chidwan3578 thanks again for the contributions. I merged all 3 commits in your PR :)