[FR] Tabbed page template

### Description

In Notion, a database page template has a modifiable layout. Which can be set to "tabbed", or left at the default ("simple").
A tabbed layout allows for having a database page have a top-level database the way a full page database also doesn't allow adding any other elements besides the database view.

The first tab of a page with tabbed layout is a content tab, which is a standard text/non database page. Any other tab added is a database. The content tab cannot be removed.

### Impact

Makes it possible to make a page template be a database page (like you can create a full page database as well elsewhere). This prevents users accidentally adding more elements or removing an inline database (database view).

It also is the only way to have tabs where one of the tabs is a text page (with full page databases the tabs can only be database views).

### Additional Context

In #8624 I was confidently wrong to think that in Notion you can create nested databases. You can't actually. In Notion you can create nested database views, but not nested databases.

That being said, the ability to nest database templates doesn't hinder anything else.