Make Web-MQTT obtain/release a FD via file_handle_cache

[lhoguin]

I have noticed that Web-MQTT does not keep track of its own FD usage,
meaning that when a Websocket connection is open, it is invisible. On
the other hand Web-STOMP connections are visible when calling
file_handle_cache:info() and counted when checking limits.

I have therefore applied a small patch to Web-MQTT to make it
behave just like Web-STOMP. I have also added a missing link
in a Web-MQTT example.

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[michaelklishin]

Backported to v3.8.x.

[michaelklishin]

Disregard my earlier comment, I see that Web STOMP is not affected 👍