Add an Antora playbook file

It's come up discussion around the migration to Antora, that the client
docs should be able to be built standalone, and not just as a part of
the larger documentation set. By adding an Antora Playbook file
(site.yml), the client docs can be built both standalone and as part of
the larger ownCloud documentation site.

See https://github.com/owncloud/client/pull/6785#issuecomment-425391756

docs/antora.yml

@@ -1,6 +1,6 @@
 name: client
 title: Desktop Client
 version: master
-
+start_page: ROOT:introduction.adoc
 nav:
 - modules/ROOT/nav.adoc

site.yml

@@ -0,0 +1,17 @@
+site:
+  title: ownCloud Client Documentation
+
+content:
+  sources:
+  - url: .
+    branches: [master-antora]
+    start_path: docs/
+
+ui:
+  bundle:
+    url: https://github.com/owncloud/docs-ui/releases/download/1.0.0/ui-bundle.zip
+  output_dir: assets
+
+output:
+  clean: true
+  dir: public