Show wiki empty state layout on wiki-only project overview page

Problem to solve

If I create a wiki-only project without a repository, the project landing page shows an empty Wiki, which is fine.
However, since we already have an awesome empty state illustration for empty wikis, we should add this to this special case as well.

Further details

• Create a new project only, disabling any permissions (project features) but Wiki
• Project landing page correctly shows the wiki, see https://gitlab.com/akaemmerle/example-project-wiki-only01
• Empty state is not following our empty state pattern

Proposal

Add the existing empty state layout and illustration to wiki-only projects, below the project header.

Design

• Illustration →
• Design specs →

Copy:

This project does not have a wiki homepage yet 

Add a homepage to your wiki that contains information about your project and GitLab will display it here instead of this message. 

Create your first page

CTA Link: /<username>/<project>/wikis/home?view=create

Links / references

• Empty state on empty wiki page
• Empty state on empty wiki on wiki-only projects