Beautify groups

What does this MR do and why?

• Hide title and filter controls when no groups are visible on a Groups page.
• Remove banner on Explore groups page. The text was used as description.
• Add illustration when there are no groups found.

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

1. Go to /admin/users and choose a user with no projects and no groups
2. Click [Impersonate]
3. Go to /dashboard/groups
4. See the first empty state
5. Go to /explore/groups
6. See no banner
7. Search something
8. See 'no results' empty state

#424397 (closed)

Closes #422076 (closed)