Theme Directory Redesign: Add a Featured or Latest Block Theme section to the main directory page

[ellenbauer]

As block themes are the foundation to use all the newest and greatest site editing features we have been working on so hard, I feel it would only make sense to use the opportunity of this redesign to highlight what we have been working on so hard over the last years.

Therefore, I propose to add a Featured or Latest Block Theme section to the theme directory page, similar as it is already done in the plugin directory.

[ryelle]

@beafialho proposed a design for this here: #13 (comment)