Part of #53322

What problem does this address?

6.3 brought in several new pages to the site editor, including pattern management, browsing pages and different detail views. This expanded further with the addition of Data Views in 6.6.

As we pursue the admin design project, we need to form a consistent way of structuring pages. This includes how we handle page headers, sub navigation, page actions, and content areas with different container sizes. This important not only internally, but for third-party plugin authors too.

What is your proposed solution?

Generally there exists the following main 'types' of admin page:

1. DataViews (e.g. Page management, Media Library)
2. Settings (e.g. Edit profile, Permalinks, and so on)
3. Dashboard
4. Generic (e.g. the WordPress About page or Site Health)

Amongst these page we find some common elements such as a title, description, primary/secondary action(s), sub-navigation, screen options, pagination, and of course the main content area.

We should design and implement reusable patterns that support these features (plus any other features deemed necessary), also accounting for responsive behavior.

It's worth noting that rudimentary, private page components already exist, and are used in DataViews. These can potentially form a base for any new components.