Implement role and user management enhancements

- Added MaxIpLimit to roles, allowing for the configuration of simultaneous IP limits for users.
- Updated role creation and update commands to include MaxIpLimit, ensuring proper handling in the application logic.
- Enhanced user management by introducing a DELETE endpoint for user accounts, with appropriate checks to prevent self-deletion.
- Updated documentation to reflect changes in role and user management, clarifying the new IP limit functionality and user deletion process.
- Adjusted related tests to cover new functionality and ensure robust validation of role and user management features.

backend/tests/PnvPanel.IntegrationTests/TestSupport/FakeXuiPanelGateway.cs

@@ -31,7 +31,7 @@ public sealed class FakeXuiPanelGateway : IXuiPanelGateway
     }
 
     public Task<Result<string>> AddClientAsync(
-        Node node, string inboundRemoteId, VpnProtocol protocol, string clientEmail, string clientName,
+        Node node, string inboundRemoteId, VpnProtocol protocol, string clientEmail, string clientName, int limitIp,
         CancellationToken cancellationToken)
         => Task.FromResult(Result.Success(Guid.NewGuid().ToString()));