Merge branch 'trigger-docs' into 'master'

Add job to trigger a docs.gitlab.com build

## What does this MR do?

Add a job to build the docs site every time master tests pass.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-com/doc-gitlab-com/issues/56

See merge request !6850

.gitlab-ci.yml

@@ -299,6 +299,14 @@ coverage:
     - coverage/index.html
     - coverage/assets/
 
+# Trigger docs build
+trigger_docs:
+  stage: post-test
+  script:
+    - "curl -X POST -F token=${DOCS_TRIGGER_TOKEN} -F ref=master https://gitlab.com/api/v3/projects/38069/trigger/builds"
+  only:
+    - master
+
 # Notify slack in the end
 
 notify:slack: