feat: Add information about debugging MQTT trafic

[jirihnidek]

• Add another section about environment variable MQTT_DEBUG

[subpop]

It looks like accepting suggestions in the PR review UI makes new commits to the branch and uses messages that aren't conformant to conventional commits. This branch will need to be rebased and fixed up before it can be merged.

[jirihnidek]

@subpop squashed and rebased.