Expand GitHub Rulesets guide with practical examples

Background

GitHub Rulesets are important for modern repository governance, but the current guide can be more practical for team leads and platform teams.

Scope

Improve 04-github-engineering/rulesets.md with examples such as:

• Branch rules for main and release/*
• Tag protection rules
• Required status checks
• Push restrictions
• Rulesets versus classic branch protection
• Common rollout strategy for existing repositories

Done When

• The guide explains when to use Rulesets.
• The guide explains when classic branch protection is enough.
• Examples are grounded in GitHub official documentation.
• Related links in README.md and section indexes still work.
• python3 scripts/check-docs.py passes.