fix: Project archive page is empty

BREAKING CHANGE: Add `layout: docs` to `content/courses/_index.md`.

Fix #1089

exampleSite/content/courses/_index.md

@@ -1,5 +1,6 @@
 ---
 title: Courses
+layout: docs  # Do not modify.
 
 # Optional header image (relative to `static/img/` folder).
 header: