move publishing to separate pipeline stage

Signed-off-by: Karel van Klink <karel.vanklink@geant.org>

.gitlab-ci.yml

 stages:
   - documentation
+  - publish
 
 include:
   # Documentation pipelines, both building and linting

docs/.gitlab-ci.yml

@@ -43,7 +43,7 @@ lint-documentation:
 
 ##### Publish documentation artefacts
 publish-documentation-1:
-  stage: documentation
+  stage: publish
   # First deploy to prod-2
   tags:
     - gap-docs-prod-2
@@ -61,7 +61,7 @@ publish-documentation-1:
     - nginx -s reload
 
 publish-documentation-2:
-  stage: documentation
+  stage: publish
   # When successful, also deploy to prod-1
   tags:
     - gap-docs-prod-1