Migrate useful content from registry/index into static-resources.md

Author: stevepiercy

docs/classic-ui/whatsnew.md

@@ -1,7 +1,8 @@
 # What's new in Plone 6 Classic UI
 
 - HTML markup is now based on [Twitter Bootstrap 5](https://getbootstrap.com/).
-- TTW theming has been removed.
+- The TTW theming editor has been removed.
+  Through-the-web theming is limited to downloading and uploading themes.
 - Twitter Bootstrap 5 icons are now included.
 - JavaScript has been modernized.
 - RequireJS has been removed.