Break up instance type documentation

[lyarwood]

/assign @lyarwood

Is your feature request related to a problem? Please describe:

The instance type documentation is starting to get a little overwhelming for users and should be broken up with a short introductory section explaining the basics.

[kubevirt-bot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

/lifecycle stale

[lyarwood]

/remove-lifecycle stale