Add dedicated User Roles page (#789)

Create a standalone User Roles reference covering all six roles (Owner,
Admin, Network Admin, Billing Admin, Auditor, User) with a permission
matrix aligned to the current dashboard, per-role sections, API/token
notes, and role-assignment steps.

Reduce the role section on the Add Users page to a pointer (keeping the
existing anchor), repoint inbound links from delete-account,
control-center, and msp-portal, add the page to the Team sidebar, and
refresh the role screenshots.

src/components/NavigationDocs.jsx

@@ -314,6 +314,7 @@ export const docsNavigation = [
             title: 'Add Users to Your Network',
             href: '/manage/team/add-users-to-your-network',
           },
+          { title: 'User Roles', href: '/manage/team/user-roles' },
           { title: 'Approve Users', href: '/manage/team/approve-users' },
           {
             title: 'Enable NetBird for Entra ID',