Add docs for tracking resize progress with observedGeneration

[singh1203]

Description

This PR updates the in-place pod resize documentation to explain how
status.observedGeneration and conditions[*].observedGeneration can be
used to track the progress of a resize request.

The new section describes:

• The meaning of status.observedGeneration and conditions[*].observedGeneration
• That these fields are populated when the PodObservedGenerationTracking
  feature gate is enabled

This addition helps users understand whether a resize is pending,
in progress, or acknowledged by the kubelet.

Closes: #51248

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: singh1203 / name: Saurabh Singh (a8d5210, dba2298)

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[singh1203]

Since the issue is already addressed #51729, I am closing this.