Document code architecture, communication between nspm manager (docker) and nspanels

[sebastienvermeille]

I had a very insightful conversation with @tpanajott this evening and have a lot to write also about denominations, key terms used in the code/project that I would like to document like:

What is a page,
What is an entity,
What is a scene,
etc.

And also I will take this opportunity to document a bit in some diagrams the communication flows between NSPM Manager <-> MQTT <-> NSPanels

So that it's more crystal clear what is going on under the hood.