feat: improve group empty state copy to explain groups/projects

Relates to issue #560765

This MR was created by Duo in Session 1167223

Changes

• Update group empty state copy to better explain what groups and projects are for new users
• Align with Pajamas empty state guidelines
• Improve terminology to reduce learning curve for users from other platforms
• Use more actionable language that defines GitLab concepts clearly

Validation steps

1. To view the standard empty state, create a top level group and visit the overview page of that group.
2. To view the empty state where you only have permission for creating subgroup, invite a user as maintainer, go to group settings groups/<your-group>/-/edit#js-permissions-settings, change Minimum role required to create projects to Owners, and make sure that Roles allowed to create subgroups is set as Maintainers.
   • Then login as that maintainer and visit the group overview page.

Screenshots

	Before	After
With only permission to create subgroup