Add a job to promote docs.airshipit.org content

A new documentation site is being added for the Airship project, so
provide a job to perform publication in the promote pipeline after
successful gating of changes.

Change-Id: I1fb3bab8902ec8266696a82d77246c346fe642d5

zuul.d/jobs.yaml

@@ -566,6 +566,17 @@
         name: afs
         pass-to-parent: true
 
+- job:
+    name: promote-airship-docs-website
+    parent: promote-tox-docs-site-base
+    description: |
+      Promote root site content for docs.airshipit.org to airshipit.org/docs.
+    final: true
+    allowed-projects:
+      - airship/treasuremap
+    vars:
+      publish_site: "airshipit.org/docs"
+
 - job:
     name: promote-governance-election
     parent: promote-tox-docs-site-base