Fix pages build

Pages build did not work as expected with PAGES_BUILD_REF. So the 'only' keyword is exchanged
by an rule. The default rule is now to build the docs only for the default branch.
PAGES_BUILD_REF is now obsoleted and removed.

ci/yocto-ktn-common.yml

 variables:
   BUILD_IMAGE_TAG: "20"
-  PAGES_BUILD_REF: master
 
 stages:
   - init
@@ -119,8 +118,8 @@ stages:
     name: kontron/docker-ktn-docs
     entrypoint: [""]
   needs: []
-  only:
-    - ${PAGES_BUILD_REF}
+  rules:
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
   script:
     - cd docs
     - mkdocs build