Qt Bluetooth: Add alt texts

qdoc: 'QtBluetooth': \image btchat-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtBluetooth': \image heartgame-start.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtBluetooth': \image heartgame-search.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtBluetooth': \image heartgame-running.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtBluetooth': \image heartgame-result.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtBluetooth': \image lowenergyscanner-services.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtBluetooth': \image lowenergyscanner-devices.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtBluetooth': \image lowenergyscanner-chars.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtBluetooth': \image peripheral-structure.png is without a textual description, QDoc will not generate an alt text for the image.