DOC-3218: Add tinymce/8 to antora-playbook.yml file. (#3730)

Author: kemister85

antora-playbook.yml

@@ -8,7 +8,7 @@ content:
     branches: HEAD
     start_path: ./
   - url: https://github.com/tinymce/tinymce-docs.git
-    branches: [ tinymce/6, tinymce/7 ]
+    branches: [ tinymce/6, tinymce/7, tinymce/8 ]
 urls:
   html_extension_style: indexify
   latest_version_segment: latest