docs(exercises): updated all exercises readme files

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

exercises/collections/README.md

@@ -1,4 +1,4 @@
-### Collections
+# Collections
 
 Rust’s standard library includes a number of very useful data
 structures called collections. Most other data types represent one
@@ -17,4 +17,6 @@ structures that are used very often in Rust programs:
   You may also know this by the names [*unordered map* in C++](https://en.cppreference.com/w/cpp/container/unordered_map), 
   [*dictionary* in Python](https://docs.python.org/3/tutorial/datastructures.html#dictionaries) or an *associative array* in other languages.
 
-[Rust book chapter](https://doc.rust-lang.org/stable/book/ch08-01-vectors.html)
+## Further information
+
+- [Storing Lists of Values with Vectors](https://doc.rust-lang.org/stable/book/ch08-01-vectors.html)