Course-overview uses a different template

Rather than overriding the overview template, just provide one with a
separate name, so that per-course overview pages can also exist in this
site.

Author: illicitonion

org-cyf/content/overview/index.md

@@ -1,4 +1,4 @@
 +++
 title = "Overview of software courses"
-layout = "overview"
+layout = "overview_of_courses"
 +++