Create 2.21 docs branch

Author: IoannisPanagiotas

doc/publish.yml

@@ -10,7 +10,7 @@ content:
     # Does not include preview (master) as it is no longer necessary.
     # When updating this, update the version in the
     # `page-add-notes-versions` below too (if relevant).
-    branches: ['2.5', '2.6', '2.7', '2.8', '2.9', '2.10', '2.11', '2.12', '2.13', '2.14', '2.15', '2.16', '2.17', '2.18', '2.19', '2.20']
+    branches: ['2.5', '2.6', '2.7', '2.8', '2.9', '2.10', '2.11', '2.12', '2.13', '2.14', '2.15', '2.16', '2.17', '2.18', '2.19', '2.20', '2.21']
     start_path: public/doc
     include: public/doc/
     exclude: