Add Antora Playbook files and update docs documentation

This commit adds two Antora Playbook files, so that the docs can be
built, stand-alone, from the remote repository and from the local
working copy. It also documents how to build the docs locally.

docs/README.md

@@ -0,0 +1,29 @@
+# Building the Docs
+
+The desktop client documentation is not built directly; instead, it is built when [the administration, developer, and user manuals](https://github.com/owncloud/docs/) are made.
+
+However, if you would like to build a local copy of the desktop client documentation, to preview changes that you are making as you are making them, you can use the following command:
+
+```
+antora \
+    --redirect-facility static \
+    --stacktrace \
+    site.local.yml
+```
+
+**Note** this command requires Antora’s command-line tools to be installed.
+To learn more about how to install them, please refer to [that documentation in the docs repository](https://github.com/owncloud/docs/blob/master/docs/install-antora.md).
+
+## Previewing the Generated Docs
+
+Assuming that there are no build errors, the next thing to do is to view the result in your browser.
+In case you have already installed a web server, you need to configure a virtual host (or similar) which points to the directory `public`, located in the root directory of the repository.
+This directory contains the generated documentation.
+Alternatively, use the [NPM Serve tool](https://www.npmjs.com/package/serve) or [PHP's built-in webserver](https://secure.php.net/manual/en/features.commandline.webserver.php).
+
+The following example starts *NPM's Serve* running in the background, using the `public` directory as its directory root, and listening on `http://localhost:5000` (if available):
+
+```
+serve public &
+```
+

site.local.yml

@@ -0,0 +1,18 @@
+site:
+  title: ownCloud Desktop Client Documentation
+
+content:
+  sources:
+  - url: .
+    branches: HEAD
+    start_path: docs/
+
+ui:
+  bundle:
+    url: https://minio.owncloud.com/documentation/ui-bundle.zip
+  output_dir: assets
+
+output:
+  clean: true
+  dir: public
+

site.yml

@@ -3,7 +3,7 @@ site:
 
 content:
   sources:
-  - url: .
+  - url: https://github.com/owncloud/client.git
     branches:
     - master-antora
     start_path: docs/