Init and update of submodules in .gitlab-ci.yml

The "Completion of the Hugo Quick Start" is an assumption, and there you install a hugo theme via git submodule, than it's necessary to add the rows I have added.

content/hosting-and-deployment/hosting-on-gitlab.md

@@ -40,6 +40,10 @@ In the root directory of your Hugo site, create a `.gitlab-ci.yml` file. The `.g
 {{< code file="gitlab-ci.yml" >}}
 image: monachus/hugo
 
+before_script:
+  - git submodule init
+  - git submodule update --force
+
 pages:
   script:
   - hugo