doc: 增加自动选择设备逻辑

Author: ArgoZhang

src/BootstrapBlazor.Server/Components/Samples/VideoDevices.razor.cs

@@ -31,6 +31,8 @@ private async Task OnRequestDevice()
             _devices.Clear();
             _devices.AddRange(devices);
             _items = [.. _devices.Select(i => new SelectedItem(i.DeviceId, i.Label))];
+
+            _deviceId = _items.FirstOrDefault()?.Value;
         }
     }