docs: add docs about stages

.gitlab-ci.yml

+# this file defines the CI/CD stages for the Ahiqar website.
+# in some cases they invoke scripts provided at .ci-scripts/ in order to
+# keep this file slim and clear. please have a look at these scripts for
+# information that goes beyond the documentation of the single stages.
+#
+# this config uses custom Docker images to keep the pipelines fast.
+# see https://gitlab.gwdg.de/mrodzis/docker-images for more info about them.
+
 cache:
   paths:
     - node_modules/
@@ -19,17 +27,13 @@ build_page:
     paths:
       - src/.vuepress/dist
 
+# this stage creates a new environment for the current branch.
 pages:
-  image: alpine:latest
+  image: docker.gitlab.gwdg.de/mrodzis/docker-images/alpine
   stage: pages
   needs:
     - job: build_page
       artifacts: true
-  before_script:
-    - apk add curl
-    - apk add jq
-    - apk add unzip
-    - apk add zip
   script:
     - ash update-artifacts.sh
     - ash remove-old-artifacts.sh
@@ -41,10 +45,9 @@ pages:
     paths:
       - public
 
+# preserves the current as well as all the old artifacts for the environments.
 pushback:
-  before_script:
-    - apk add zip
-  image: alpine:latest
+  image: docker.gitlab.gwdg.de/mrodzis/docker-images/alpine
   stage: preserve-pages
   needs:
     - job: pages
@@ -56,11 +59,9 @@ pushback:
     paths:
       - updated-artifact.zip
 
+# removes the environments that are no longer needed.
 remove_envs:
-  before_script:
-    - apk add curl
-    - apk add jq
-  image: alpine:latest
+  image: docker.gitlab.gwdg.de/mrodzis/docker-images/alpine
   stage: remove_unused_envs
   needs:
     - job: build_page