Rename exercises dir

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

topics/shell/solutions/basic_date.md

@@ -0,0 +1,13 @@
+## Basic Date
+
+### Objectives
+
+1. Write a script that will put the current date in a file called "the_date.txt"
+
+### Solution
+
+```
+#!/usr/bin/env bash
+
+echo $(date) > the_date.txt
+```