Issue #3594422: Add a Tugboat preview configuration
Fixes #3594422.
Adds .tugboat/config.yml + .tugboat/setup.php (mirroring audit_trail and pdv). Stands up a single-site preview:
- Enables webdav + admin_toolbar.
- Adds a file field to Article rendered with the
webdav_linkformatter (the 'Open over WebDAV' links). - Creates a demo user (webdav_user / webdav_user) with the access + modify WebDAV permissions, and a sample Article with a document attached.
- Guide front page: how to connect a WebDAV client (Finder, Windows Explorer, LibreOffice, cadaver) and how the per-file URL-token auth works.
- Rebuilds daily.
WebDAV is a desktop-client protocol, so the file verbs happen in a client, not the browser; the preview gives everything needed to point a client at the site. Tugboat must also be enabled for the project in its Drupal.org settings for previews to run.