[DOCS] Theengs Plug highlight and SEO improvements (#1730)

* [DOCS] Theengs Plug highlight

* Add alt content for images and description

docs/prerequisites/board.md

@@ -21,7 +21,7 @@ If you want to use the BLE decoding capabilities of OpenMQTTGateway with a table
 * Energy consumption monitoring,
 * Presence detection (beta).
 
-[![](../img/Theengs-Plug-OpenMQTTGateway.png)](https://shop.theengs.io/products/theengs-plug-smart-plug-ble-gateway-and-energy-consumption)
+[![Theengs plug view](../img/Theengs-Plug-OpenMQTTGateway.png)](https://shop.theengs.io/products/theengs-plug-smart-plug-ble-gateway-and-energy-consumption)
 
 Support the project by purchasing the [Theengs plug](https://shop.theengs.io/products/theengs-plug-smart-plug-ble-gateway-and-energy-consumption)
 The plug is available in North America only, other regions are planned.