Added content about Behavioral Design Patterns

[paridhi26]

• added content about Behavioral Design Patterns in System Design Folder (Topics/System_design/Behavioral_Design_Patterns.md).

• explained the usefulness of behavioral design patterns and explained some common behavioral patterns using intuitive real world analogies.

• edited Topics/System_Design.md to add a link to my newly added content file Behavioral_Design_Patterns.md.

[paridhi26]

overall

Good job so far! There would be no merge conflicts for your PR. However, there are two aspects I would like it to be addressed in your contribution regarding the topic of behavioral design patterns:

• Make sure you show a little implementation via code for each of your Observer, Strategy, Iterator, and Command design patterns.
• Make sure you mention the pros and cons of each of the design patterns you are talking about

Also, make sure the images appear in your contribution. Right now the images do not appear at least for me.

Overall, I like how you covered the scenarios you mentioned for each design pattern. I like how you thoroughly defined each of the design patterns you are discussing.

Thank you Mahan for the valuable review comments! I am fixing the image issue now. For this content page, I am not adding any code snippets or implementation details since they are available on many websites. I want this content page to just provide real world analogies and examples about usage of different behavioural patterns.