Rename exercises dir

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

topics/cicd/remove_builds.md

@@ -0,0 +1,14 @@
+### Jenkins - Remove Jobs
+
+#### Objective 
+
+Learn how to write a Jenkins script that interacts with builds by removing builds older than X days.
+
+#### Instructions
+
+1. Pick up (or create) a job which has builds older than X days
+2. Write a script to remove only the builds that are older than X days
+
+#### Hints
+
+X can be anything. For example, remove builds that are older than 3 days. Just make sure that you don't simply remove all the builds (since that's different from the objective).