Mqtt integration

[mghill]

Implementation of #16

Could be enriched with images, but not strongly dependent on it.

[kimburgess]

Purely notes on semantic and structural elements. Will review content separately.

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[mghill]

added the <abbr> tags as discussed above, as well as a test of image hosting, and a proof of concept of Mermaid.js but it's very janky there because it was written in their online editor and then embedded remotely rather than written inline as intended. With that caveat though, it works pretty well.

[mghill]

As I see it this is currently soft blocked by 2 things.

1. Sidebar categories issue Sidebar Customisation docs-site#20
   i) this should go in Overview, in a subfolder for something like protocols but I'm not clear on what our current stance is on that. It could go in plain for now, but that starts getting cluttered and it is usually better to sort tooling first
2. Mermaid.js integration Add support for diagram rendering docs-site#14
   i) I don't know if this is "down the track" enough that we should just use the preexisting static image at the top. There is a proof of concept Mermaid there currently (though it doesn't look great, and isn't actually generated inline - this one is in their online editor)