[doc] Metal3 quickstart Issue in "Prerequisites"

[joesusecom]

Prerequisites:

https://suse-edge.github.io/quickstart-metal3.html#id-prerequisites

Without any further background, this section is a bit unclear. We're introducing the "Management cluster" for the first time without explaining what it is.

Maybe this gets easier once a diagram is added. But I'd still explain "Management cluster".

"control plane" as two words.

[hardys]

I'll close this one - please comment on #170 if you think it still needs improvement after the diagram is added for context

[hardys]

Actually I'll reopen and we can track resolution via #170