Add diagram to home page to visualize role of machinekit

[JohnAD]

"A picture is worth a thousand words"

This PR adds a image to the main page of machinekit to visually describe what machinekit does and it's placement in relation to other parts. The goal is to help new users see the big picture.

It shows the UI as separate (even though machinekit does have some example UIs); and it shows the relationship of HAL to the actual equipment.

To the power users of machinekit: is the diagram accurate?

I'm not really an artist, so if someone can make a similar diagram that looks better; I will not be offended. Replace away!

[cerna]

@JohnAD The button with "STOP" inscription which is connected by solid line to the "Controller I/O" with help "HAL UI" gives the idea that it is Emergency STOP button, normally known as an E-STOP button. That cannot be, because estop has to be deterministic and the HAL remote communication does not satisfy this requirement.

[JohnAD]

Good point. And, I also removed the dotted line to the API as I know of no E-stop buttons that have any kind of IP connectivity. I'm now showing the stop button wired to both the machine (for shutting down) and the controller board (for E-stop status).

[luminize]

thanks @JohnAD