Add (mermaid.js) diagrams to the docs that show system architecture

[MattTriano]

From the material-for-mkdocs diagrams doc page, it looks like it will be a very smooth integration.

And from the mermaid.js docs, it looks like mermaid.js makes it unbelievably easy to create extremely expressive diagrams purely in simple syntax in markdown files.

Examples of that expressiveness:

• git commit flows (git log --graph)
• Gantt charts
• Mindmaps
• DAGs/DCGs/Flow charts
• Sequence diagrams (good for TCP)
• ER Diagrams
• State diagrams
• regular old UML diagrams
• and more!

This looks like a really cool tool.