Draft: Update empty wiki homepage

What does this MR do and why?

• Migrate promo block to Pajamas components

Next step: show file tree + readme as fallback below the banner/alert. It causes some errors, so I'll do it in a separate MR.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Banner for maintainer+

1. Go to http://gdk.test:3000/-/profile/preferences
2. Scroll to Project overview content and choose Wiki
3. Go to http://gdk.test:3000/flightjs/Flight or any other project you can edit

Banner for a visitor

1. Go to http://gdk.test:3000/admin/users/reported_user_1
2. Click [Impersonate]
3. Go to http://gdk.test:3000/-/profile/preferences
4. Scroll to Project overview content and choose Wiki
5. Go to http://gdk.test:3000/flightjs/Flight or any other public project you can't edit

#452225 (closed)

Closes #442543