[v3] Extract Concepts from v2 exercise: collatz-conjecture

Fixes #401

Author: vaeng

reference/exercise-concepts/collatz-conjecture.md

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+# Collatz Conjecture
+
+## Task
+
+The Collatz Conjecture or 3x+1 problem can be summarized as follows:
+
+Take any positive integer n. If n is even, divide n by 2 to get n / 2. If n is odd, multiply n by 3 and add 1 to get 3n + 1. Repeat the process indefinitely. The conjecture states that no matter which number you start with, you will always reach 1 eventually.
+
+Given a number n, return the number of steps required to reach 1.
+
+## Concepts
+
+- .h file: the most common approach is to use a `.h` for the declarations
+- .cpp file: the most common approach is to use a `.cpp` for the definitions
+- include guard: protect against multiple inclusion within a single translation unit
+- .cpp includes .h: a `.cpp` file should include its `.h` file
+- namespaces: the function is member of a namespace
+- functions: used as the main entry point for the exercise
+- including directives: `<stdexcept>` is needed for `std::domain_error`
+- function arguments: starting number
+- return values: the result is returned by the function
+- conditionals using if: conditionally execute logic using an `if` statement
+- comparison operators: `!=` and `<=`
+- exceptions: throw an exception if the starting number is smaller than 1
+- signed integers: the core guidelines recommend signed integers for arithmetic
+- increment: `++`
+- math operators: the modulo operator `%` is used
+- loops: use `while` to check if function has reached its end
+
+## Alternative approaches
+
+- implement solution as `inline` functions in the `.h` file
+- using a recursive solution and call `steps` every time the end-condition has not been met