DOC-3218: Add tinymce/8 to branch to antora-playbook.yml for v6. (#3729)

Author: kemister85

antora-playbook.yml

@@ -8,7 +8,7 @@ content:
     branches: [ HEAD ]
     start_path: ./
   - url: https://github.com/tinymce/tinymce-docs.git
-    branches: [ tinymce/5, tinymce/7 ]
+    branches: [ tinymce/5, tinymce/7, tinymce/8 ]
 urls:
   html_extension_style: indexify
   latest_version_segment: latest