Rough rework of the CI setup documentation (#12094)

.gitlab-ci.yml

@@ -1,9 +1,9 @@
 ---
 stages:
-  - build
-  - test
-  - deploy-part1
-  - deploy-extended
+  - build # build docker image used in most other jobs
+  - test # unit tests
+  - deploy-part1 # kubespray runs - common setup
+  - deploy-extended # kubespray runs - rarer or costlier (to test) setups
 
 variables:
   FAILFASTCI_NAMESPACE: 'kargo-ci'