docs(exercises): updated all exercises readme files

all exercises readme files now have a unified structure and a description

exercises/structs/README.md

@@ -1,8 +1,8 @@
-### Structs
+# Structs
 
 Rust has three struct types: a classic C struct, a tuple struct, and a unit struct.
 
-#### Book Sections
+## Further information
 
 - [Structures](https://doc.rust-lang.org/book/ch05-01-defining-structs.html)
 - [Method Syntax](https://doc.rust-lang.org/book/ch05-03-method-syntax.html)