add playbook template to support local build in content branches (#12497)

lib/antora/templates/per-branch-antora-playbook.yml

@@ -0,0 +1,19 @@
+# The purpose of this Antora playbook is to generate a preview of the docs in the current branch.
+site:
+  title: Spring Security Reference
+content:
+  sources:
+  - url: ./..
+    branches: HEAD
+    start_path: docs
+    worktrees: true
+asciidoc:
+  attributes:
+    page-pagination: ''
+    hide-uri-scheme: '@'
+urls:
+  latest_version_segment: ''
+ui:
+  bundle:
+    url: https://github.com/spring-io/antora-ui-spring/releases/download/latest/ui-bundle.zip
+    snapshot: true