docs: comprehensive README documentation update

- Fix broken documentation links to non-existent files
- Remove invalid 'populate' option from contentTypes config examples
- Document all 12 helper functions with examples
- Add complete Monitoring Service documentation
- Add Entity Subscription helpers documentation
- Clarify admin panel structure and widget requirements (Strapi v5.13+)
- Update types.d.ts with entity subscription interfaces
- Update api-reference.json with new methods
- Update docs/api/io-class.md with entity subscription section
- Fix version references (3.x -> 5.x)

Author: Schero94