W-23997997: Remove on-premises from the control-plane compatibility matrix

[kevintroller]

Summary

• Replaces #486, which could not merge after Control Planes #487 landed on latest.
• The On-Premises row is already gone from the matrix on latest. This PR keeps Beth's intro: on-premises isn't listed because it does not require a control plane; registering the Runtime Manager agent makes that instance Hybrid Standalone.
• Points to xref:mule-runtime::mule-standalone.adoc[Run Mule Runtime Engine On-Premises].

Test plan

• Hosting Options matrix still has four runtime rows (CH2, CloudHub, RTF, Hybrid Standalone Runtimes).
• Compatibility intro explains why on-premises is omitted and how it relates to Hybrid.
• The xref to Run Mule Runtime Engine On-Premises resolves.