docs: add UnitedDeployment v1beta1 promotion proposal

[Kunal1522]

Summary

• add a proposal for UnitedDeployment v1beta1 promotion
• focus on API contract, validation/defaulting, rollout semantics, and promotion plumbing
• include review questions for the remaining beta decisions

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[Copilot]

Pull request overview

Adds a design/proposal document to guide promoting UnitedDeployment from v1alpha1 to v1beta1, focusing on API contract clarifications and cleanup items needed prior to serving the beta API.

Changes:

• Introduces a promotion proposal covering API surface gaps (e.g., subsets[].patch contract, status field ownership).
• Documents validation/defaulting inconsistencies and rollout/revision semantics that need resolution before v1beta1.
• Lists concrete promotion plumbing steps (new v1beta1 types, conversions, CRD serving changes, tests, and migration notes) plus review questions.

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[Copilot]

Several references include specific line ranges in the link text (e.g., uniteddeployment_types.go:220-226), but the Markdown link target points only to the file path without a #Lx-Ly anchor. This makes the line-range references non-clickable/misleading; consider adding GitHub line anchors (or removing the line-range from the visible text) consistently throughout the doc.

Suggested change

	- [`uniteddeployment_types.go:220-226`](../../apis/apps/v1alpha1/uniteddeployment_types.go)
	But the adapters apply it to `spec.template` only:
	- [`statefulset_adapter.go:170-184`](../../pkg/controller/uniteddeployment/adapter/statefulset_adapter.go)
	- [`advanced_statefulset_adapter.go:184-199`](../../pkg/controller/uniteddeployment/adapter/advanced_statefulset_adapter.go)
	- [`cloneset_adapter.go:149-163`](../../pkg/controller/uniteddeployment/adapter/cloneset_adapter.go)
	- [`deployment_adapter.go:164-179`](../../pkg/controller/uniteddeployment/adapter/deployment_adapter.go)
	- [`uniteddeployment_types.go:220-226`](../../apis/apps/v1alpha1/uniteddeployment_types.go#L220-L226)
	But the adapters apply it to `spec.template` only:
	- [`statefulset_adapter.go:170-184`](../../pkg/controller/uniteddeployment/adapter/statefulset_adapter.go#L170-L184)
	- [`advanced_statefulset_adapter.go:184-199`](../../pkg/controller/uniteddeployment/adapter/advanced_statefulset_adapter.go#L184-L199)
	- [`cloneset_adapter.go:149-163`](../../pkg/controller/uniteddeployment/adapter/cloneset_adapter.go#L149-L163)
	- [`deployment_adapter.go:164-179`](../../pkg/controller/uniteddeployment/adapter/deployment_adapter.go#L164-L179)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.77%. Comparing base (530f9c9) to head (3143f4e).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2392      +/-   ##
==========================================
+ Coverage   48.68%   48.77%   +0.09%     
==========================================
  Files         324      324              
  Lines       27920    27928       +8     
==========================================
+ Hits        13592    13623      +31     
+ Misses      12787    12775      -12     
+ Partials     1541     1530      -11     

Flag	Coverage Δ
unittests	48.77% <ø> (+0.09%)	⬆️

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