Hide main page content block from XB

This is a new requirement; hence cannot be a bug. This relates to the whole "content region should never be made editable in XB and should always be assumed to contain only the main content block" thing that you wrote in the issue queue, but which was absent from both requirements and designs.

IMHO this should be merged with 📌 Consider refactoring page_template into page_region(s) Active , see explanation in #3501600-7: Consider splitting 1 PageTemplate config entity into N PageRegion config entities → and subsequent comment.