Feature: Add Design Patterns Page

[oveldman]

I want to try this kind of patterns:

1. The Observer Pattern 🟢
2. The Decorator Pattern 🟢
3. The Factory Pattern 🟢
4. The Singleton Pattern 🟢
5. The command Pattern 🔴
6. The Adapter 🔴
7. The Facade Pattern 🔴
8. The Template Method Pattern 🔴
9. The Iterator 🔴
10. The Composite Pattern 🔴
11. The State Pattern 🔴
12. The Proxy Pattern 🔴
13. Compound Patterns 🔴

TODO: 🔴
Done: 🟢