QtNetwork: Add alt texts

qdoc: 'QtNetwork': \image blockingfortuneclient-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image broadcastreceiver-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image broadcastsender-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image dnslookup-example.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image http-example.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image multicastreceiver-example.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image multicastsender-example.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image multi-stream-client-example.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image multi-stream-server-example.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image network-chat-example.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image rsslisting.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image securesocketclient-example.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image threadedfortuneserver-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image torrent-example.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtNetwork': \image network-examples.webp is without a textual description, QDoc will not generate an alt text for the image.