Describe your motivation

#8885 added a root level heading API to <vaadin-dashboard>. We should support the same feature in <vaadin-dashboard-layout>.

Describe the solution you'd like

There have been two options discussed so far:

1. Provide the same API, where the root heading level can be configured on the dashboard layout. Widgets and sections added to the layout would then automatically determine their heading level from the closest dashboard layout when they are added to the DOM, and observe changes to the root heading level while they are connected to the DOM. Ideally the same mechanism would be used in both <vaadin-dashboard> and <vaadin-dashboard-layout>.
2. If 1. is not feasible, provide an API for configuring an individual heading level for each widget or section.

Describe alternatives you've considered

No response

Additional context

No response