Improve wiki empty state messages

Problem to solve

The empty state messages for wikis mention "project" in a few places, we'd like to generalize this a bit to avoid duplicated translations for group wikis.

There are three different states depending on user access:

1: Write access	2: Write access, and permission to create issues	3: Read-only access

Proposal

@mayra-cabrera suggested using "project or group", which I think would be great if we can avoid repeating it.

Maybe we can rephrase the titles (other suggestions welcome):

• 1: The wiki lets you collaborate on documentation?
• 2/3: This wiki doesn't have any content yet?

Further details

Links / references

Follow-up from !34079 (comment 358217851).