CA DRA: document DRA autoscaling

[towca]

Which component are you using?:

/area cluster-autoscaler
/area core-autoscaler
/wg device-management

Is your feature request designed to solve a problem? If so describe the problem this feature should solve.:

We don't have any documentation for DRA autoscaling, we should close that gap.

Describe the solution you'd like.:

We should document the feature on the Node autoscaling page in k8s docs.

Additional context.:

This is a part of Dynamic Resource Allocation (DRA) support in Cluster Autoscaler. An MVP of the support was implemented in #7530 (with the whole implementation tracked in kubernetes/kubernetes#118612). There are a number of post-MVP follow-ups to be addressed before DRA autoscaling is ready for production use - this is one of them.