Finish adding sheep pictures

[harsh183]

So far we only have a few but many exist and many more were planned. Let's add all of them.

[harsh183]

I added all the older ones we didn't have and fixed the includes issue.

• [ ] Loops
• [ ] Classes and Objects
• [ ] Data Classes
• [ ] Inheritance
• [ ] Generics

Depending on how the ktor and coroutine stuff comes along we can order a new batch of illustrations. For now let's finish all the core articles.

[harsh183]

I'm thinking of more from @astha183

• [ ] Val vs var
• [ ] Type inference
• [ ] Null safety
• [ ] How to read tutorial page
• [ ] Basics of testing
• [ ] Common picture for all coroutine related
• [ ] Common picture for all ktor related