Switch to core page template modal

[simison]

We built a "new page template" ahead of core, which was good:

Since then, core has been building similar modal over time, but it was still missing categories when I last time checked in Dec 2022 (p58i-dox-p2#comment-56492).

The modal is in active development again and will likely match or pass the UX of our soon:

We should switch to core modal when core modal has categories sidebar.

If other flow improvements from core issue ship first, we should ensure our modal works well with those improvements:

Important to follow up with docs updates.

FYI @autumnfjeld @ianstewart