docs: add rules reference page

[leno23]

Summary

• Add docs/rules.md as the requested rules reference page with all 16 rules.
• Include severity, category, rationale, bad/good examples, suggested fix text, and source links for each rule.
• Link the new reference page from the README rules section.

Fixes #1

Test plan

• Static validation: generated reference contains 16 unique rule sections.
• Static validation: each section contains source link, bad example, good example, and suggested fix text.
• Static validation: README now links to docs/rules.md.
• Not run: runtime test suite, because this is a documentation-only change.