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

* [DOCS] Theengs Plug highlight

* Add alt content for images and description

docs/prerequisites/M5.md

@@ -17,14 +17,14 @@ The display is always OFF, if the board has a led, the led is used when processi
 To change the low power mode you can do it by [MQTT](../use/boards#low-power-mode-for-m5-boards) or by pushing the middle button (btn B) of the M5Stack and the top button (btn B) of the M5Stick C
 
 ### Start screen
-![](../img/OpenMQTTgateway_M5_Stack_Board_Display_Logo.png)
+![MStack core logo](../img/OpenMQTTgateway_M5_Stack_Board_Display_Logo.png)
 
 ### Sleep screen
 The screen brightness is put to a minimum so as to reduce power consumption.
-![](../img/OpenMQTTgateway_M5_Stack_Board_Display_Text.png)
+![M5Stack core brightness](../img/OpenMQTTgateway_M5_Stack_Board_Display_Text.png)
 
 ### Log display in case of warning
 If the gateway detects a warning or an error the sleep go out the sleep screen and show the message with a red (error) or orange (warning) background.
-![](../img/OpenMQTTgateway_M5_Stack_Board_Display_Warning.png)
+![M5 Stack core warning](../img/OpenMQTTgateway_M5_Stack_Board_Display_Warning.png)
 
 The M5Stick support low power mode 2 but its display is not supported for the moment.

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.