MTM-64962 Rework MQTT Service overview for GA

[scmi-c8y]

Still a work in progress but ready for a first round of review. The basic approach is to pitch the MQTT Service as a unified MQTT endpoint that supports Cumulocity as well as generic device protocols, and that is optimised for device integration use cases. It talks about some things that don't yet exist but that should be there in the GA. Some points are repeated several times (e.g., device isolation) in the hope that readers will notice them at least once. I removed the "MQTT Service vs. Core MQTT" table because most of it was covered elsewhere, and we don't want to pitch them against each other so much now that the service also supports the core protocols.

Note: This PR targets an integration branch where we can collect all the MQTT Service GA docs for publishing together.

[github-actions]

Preview available here

[rthrippleton]

Begs the question - because we're having to put in a navigation 'guide' here, doesn't that mean our section headings could be better? Not everybody reads doc linearly, so if somebody can locate the relevant section just from the tree on the left, that'd be better.

My modest and hastily written suggestions:

• Key features and high-level architecture
• Supported MQTT versions and features
• Integrating MQTT devices with the service
• Developing clients to communicate with MQTT devices

[bbyreddy]

Nitpick:
In the MQTT service introduction section, we refer mention "Connecting MQTT devicesshows how to integrate devices with the service, using either the Core MQTT protocols or generic MQTT messaging."
Just to keep it consistent, we can say "Core MQTT or generic MQTT device protocols". We use "generic MQTT device protocol" in the "Protocols and clients" table.

"device message format" and "device protocols" are used synonymously. Possibility to unify?