KP-7378 Only deploy from main

This way we won't overwrite the production website with unfinished
content that has not been reviewed yet. Left the extra rows as comments,
because enabling them allows one to provision the full website on their
own fork (and thus in myuser.github.io/Kielipankki instead of on the
official course material site).

.github/workflows/gh-pages.yml

@@ -1,14 +1,18 @@
 name: Deploy GitHub Pages, compiling slides and tutorials/handouts to HTML
 
 on:
-  pull_request:
-    types:
-      - opened
-      - reopened
-      - synchronize
   push:
     branches:
       - main
+# If you want to test these (*in a forked repository* to avoid overwriting the
+# real production website), you can enable these rows. In that case you also
+# need to run a Runner yourself, see instructions in
+# https://github.com/CSCfi/kielipankki-actions-pipeline/blob/main/runners/README.md
+#  pull_request:
+#    types:
+#      - opened
+#      - reopened
+#      - synchronize
 
   # Allows you to run this workflow manually from the Actions tab
   workflow_dispatch: