Dataviews layouts should have navigable regions

[tellthemachines]

Description

When the site editor is open in pages or templates view, there are only two or three regions on the screen: "Navigation" (the left sidebar with a black background), "Pages" or "Templates" (the dataview) and, if in List layout, "Editor content".

The issue is that, within the data view, there are header, main content and footer areas that would benefit from being regions themselves:

Step-by-step reproduction instructions

1. Open the site editor and navigate to Templates;
2. Use Ctrl + backtick to navigate between regions and verify there are only two of them.

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure