Add National Grid US virtual integration docs (#38766)

Co-authored-by: c0ffeeca7 <38767475+c0ffeeca7@users.noreply.github.com>

source/_integrations/national_grid_us.markdown

@@ -0,0 +1,20 @@
+---
+title: National Grid US
+description: Get energy usage from National Grid US using the Opower integration
+ha_category:
+  - Energy
+  - Sensor
+ha_release: 2025.5
+ha_domain: national_grid_opower
+ha_integration_type: virtual
+ha_supporting_domain: opower
+ha_supporting_integration: Opower
+ha_codeowners:
+  - '@tronikos'
+ha_config_flow: true
+ha_platforms:
+  - sensor
+ha_iot_class: Cloud Polling
+---
+
+{% include integrations/supported_brand.md %}