Add the proper empty state when wiki page not found

What does this MR do and why?

Problem: we say "Wiki doesn't exist" when the page doesn't exist but not the whole Wiki.

What was done here:

• Add the proper empty state when wiki page not found
• Show sidebar navigation

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Before	After

How to set up and validate locally

1. Create a wiki for any public project
2. Go to http://gdk.test:3000/admin/users/reported_user_1 or any other user with no groups and projects
3. Click [Impersonate]
4. Go to your created wiki
5. Change the page name in the url
6. See empty state page
7. Don't forget to stop impersonating by clicking the icon near your avatar

#452225 (closed)

Fixes #22319 (closed)

Fixes #21891 (closed)