Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/os/fork_101.md

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+## Fork 101
+
+Answer the questions given the following program (without running it):
+
+```
+#include<stdio.h>
+#include <unistd.h> 
+int main()
+{
+  fork();
+  printf("\nyay\n");
+  return 0;
+}
+```
+
+1. How many times the word "yay" will be printed?
+2. How many processes will be created?

topics/os/fork_102.md

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+## Fork 101
+
+Answer the questions given the following program (without running it):
+
+```
+#include<stdio.h>
+#include <unistd.h> 
+
+int main()
+{
+  fork();
+  fork();
+  printf("\nyay\n");
+  return 0;
+}
+```
+
+1. How many times the word "yay" will be printed?
+2. How many processes will be created?

topics/os/solutions/fork_101_solution.md

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+## Fork 101 - Solution
+
+1. 2
+2. 2

topics/os/solutions/fork_102_solution.md

@@ -0,0 +1,4 @@
+## Fork 102 - Solution
+
+1. 4
+2. 4