📖 Add v1alpha4 to v1beta1 upgrade doc

[sbueringer]

What this PR does / why we need it:
This PR adds the initial version of the upgrade doc.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #5262

[fabriziopandini]

/lgtm

[killianmuldoon]

Is this type of change to the contract a worthwhile inclusion: #4949 ?

[fabriziopandini]

Is this type of change to the contract a worthwhile inclusion: #4949 ?

This is already included in v1alpha4, so no need to document in the v1alpha4 --> v1beta1 doc; however, generally speaking,contract changes should be documented in this doc.

[randomvariable]

Is it worth adding a comment about potentially supporting conversions from v1alpha3 as well as v1alpha4 to the storage version?

[killianmuldoon]

This is already included in v1alpha4, so no need to document in the v1alpha4 --> v1beta1 doc; however, generally speaking,contract changes should be documented in this doc.

So should that change be included in the v1alpha3 -> v1alpha4 docs?

[sbueringer]

This is already included in v1alpha4, so no need to document in the v1alpha4 --> v1beta1 doc; however, generally speaking,contract changes should be documented in this doc.

So should that change be included in the v1alpha3 -> v1alpha4 docs?

This change was introduced during v1alpha4, not from v1alpha3->v1alpha4. I don't think that providers will re-read the v1alpha3=>v1alpha4 migration doc once they migrated to v1alpha4.

Is it worth adding a comment about potentially supporting conversions from v1alpha3 as well as v1alpha4 to the storage version?

I think that makes sense, if the idea is that infra providers also potentially support the migration from v1alpha3=>v1beta1. I added a section about it, I assume it's optional for providers to support it.

[fabriziopandini]

/lgtm
/approve

Let document retroactively #4949 on v1alpha3 -> v1alpha4 doc + the need of implementing InfrastructureClusterTemplate or ControlPlaneTemplate for ClusterClass support

[k8s-ci-robot]

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