CI: only deploy pages from the main branch.

.gitlab-ci/pages.yml

@@ -24,10 +24,13 @@ pages:
     - mdbook build . -d public
     - mv public ..
   rules:
-    - changes:
+    - if: '$CI_COMMIT_REF_NAME == "main"'
+      changes:
         - doc/**/*
         - src/bin/doc.rs
         - .gitlab-ci/pages.yml
+    - if: '$CI_COMMIT_REF_NAME != "main"'
+      when: manual
   artifacts:
     paths:
       - public