docs(resources): add learning cheat sheets and security guidelines

[wendev27]

Summary

This pull request expands the DevSecOps Learning Lab resources by adding cheat sheets, reference materials, and security guidelines gathered throughout Phases 1–3.

Added

• Phase 1 Foundations cheat sheet
• Phase 2 Containerization cheat sheet
• Phase 3 CI/CD cheat sheet
• Security sharing guidelines for screenshots and terminal outputs
• Additional reference materials for future checkpoints

Purpose

The goal of these resources is to build a personal knowledge base that documents key concepts, commands, mental models, and best practices learned during the DevSecOps journey.

These materials will serve as quick references for future topics such as reverse proxies, Kubernetes, observability, and cloud infrastructure.

Notes

• No application logic was changed.
• No infrastructure changes were introduced.
• This update focuses on documentation and long-term learning resources.