feat(solutions): add sorting-algorithms/c/insertion-sort

challenges/sorting-algorithms/solutions/c/insertion-sort/README.md

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+# sorting-algorithms/c/insertion-sort
+
+Created by [@Divlo](https://github.com/Divlo) on 29 June 2021.
+
+| Algorithm                                                   | Best Case   | Average Case | Worst Case  |
+| ----------------------------------------------------------- | ----------- | ------------ | ----------- |
+| [Insertion sort](https://wikipedia.org/wiki/Insertion_sort) | O(n)        | O(n²)        | O(n²)       |

challenges/sorting-algorithms/solutions/c/insertion-sort/insertion_sort.h

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+#ifndef INSERTION_SORT_H
+#define INSERTION_SORT_H
+
+void insertion_sort(int *numbers, const int length);
+
+#endif

challenges/sorting-algorithms/solutions/c/insertion-sort/solution.c

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+#include <stdio.h>
+#include <stdlib.h>
+
+#include "insertion_sort.h"
+
+int main() {
+  int *numbers = malloc(25000 * sizeof(int));
+  int current_number;
+  int index_input = 0;
+  while (scanf("%d", &current_number) != EOF) {
+    numbers[index_input] = current_number;
+    index_input += 1;
+  }
+  insertion_sort(numbers, index_input);
+  for (int index = 0; index < index_input; index++) {
+    printf("%d\n", numbers[index]);
+  }
+  free(numbers);
+  return 0;
+}
+
+void insertion_sort(int numbers[], const int length) {
+  for (int index_1 = 1; index_1 < length; index_1++) {
+    int current = numbers[index_1];
+    int index_2 = index_1 - 1;
+    while (index_2 >= 0 && numbers[index_2] > current) {
+      numbers[index_2 + 1] = numbers[index_2];
+      index_2 -= 1;
+    }
+    numbers[index_2 + 1] = current;
+  }
+}