Create mentoring.md for grains exercice in cpp

Creating the guide for the solution of the grains exercise in cpp. Offering an imperative and a recursive solution for the first function and explaining how to do correctly the second function.

tracks/cpp/exercises/grains/mentoring.md

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+# Mentoring
+
+## First function
+
+The first way to implement this function is to use a for loop :
+```cpp
+unsigned long long square(int indice) {
+    unsigned long long result = 1;
+    for (int i = 0; i < indice; i++) result *= 2;
+    return result;
+}
+```
+A cleaner way to do this is to use a recursive function :
+```cpp
+unsigned long long square(int indice) {
+    if (indice == 1) return 1;
+    else return square(indice - 1) * 2;
+}
+```
+This way, the code is shorter and in my mind, easier to read.
+
+## Second function
+At first sight, we want to use the first function to do it easily :
+```cpp
+unsigned long long total() {
+    unsigned long long sum = 0;
+    for (int i = 1; i <= 64; i++) sum += square(i);
+    return sum;
+}
+```
+However with this solution, each time you call the function square with the indice i, you do i products.
+In the following solution, you only do 64 products and by adding to the sum each time you multiply by 2 :
+```cpp
+unsigned long long total() {
+    unsigned long long total = 0;
+    unsigned long long square = 1;
+
+    for (int k = 0; k < 64; k++) {
+        total += square;
+        square *= 2;
+    }
+
+    return total;
+}
+```