Doc: Move `Class overview` section somewhere more prominent

[LecrisUT]

Particularly on the page, Phases and Steps parts should be made more visible so that users can more easily know:

• The steps order
• Which steps are guestless/guestful

I often get tripped up when I forget to add a discover step and I don't see anything running. Another example is from #3065 where it is easier to visualize where to put the container setup steps, if they know the step order and the concepts of guestless/guestful it helps intuit provision and prepare steps.