Expand on MQTT Plugin documentation.

[rhuss]

Extended plugin documentation for MQTT plugins and added missing fields. Most of the information is extracted from the source.

Adds also more context to the YAML examples as mentioned in #664

[rhuss]

How is the English translation created? Is it done manually or by some tooling? If done manually, I'm happy to translate the extra content.

Also, it might make sense to extract the pipeline-specific configuration that applies to other plugins (like the HTTP plugin) into a separate section so that it can be referenced from the MQTT and HTTP plugin documentation.

[rhuss]

Another question: For the writing case - what variables are available in which context? The current example is not helpful as it only mentions var, which is just a placeholder. Later in the docs, I also found enable in an example. What is the list, and what can be used in the templates as variables? It's a bit confusing.