Issue #3527319 by bluegeek9: Ability to preview GitLab Pages in CI

Closes #3527319

.gitlab-ci.yml

@@ -54,6 +54,10 @@ variables:
   OPT_IN_TEST_MAX_PHP: 1
   _PHPUNIT_CONCURRENT: 0
 
+.skip-pages-rule: &skip-pages-rule
+  if: '$SKIP_PAGES == "1"'
+  when: never
+
 phpunit (coverage):
   extends: phpunit
   rules:
@@ -85,15 +89,35 @@ phpunit:
   rules:
     - when: never
 
+# Overriding rules for the "pages" job so it runs for MRs.
 pages:
-  stage: test
+  rules:
+    - *skip-pages-rule
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - exists:
+        - mkdocs.yml
+      when: on_success
+
+pages-preview:
+  stage: deploy
   needs:
-    - job: phpunit (coverage)
+    - job: pages
+      artifacts: true
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+  artifacts:
+    expose_as: 'MkDocs Preview'
+    paths:
+      - 'public/index.html'
+  pages:
+    path_prefix: "preview-$CI_MERGE_REQUEST_IID"
   before_script:
     - cp $CI_PROJECT_DIR/favicon.ico $CI_PROJECT_DIR/docs/favicon.ico
     - cp $CI_PROJECT_DIR/README.md $CI_PROJECT_DIR/docs/index.md
     - cp -R $CI_PROJECT_DIR/coverage $CI_PROJECT_DIR/docs/coverage
 
+
 ###################################################################################
 #
 #                                        *