Document v1alpha4 -> v1beta1 changes for providers in CAPI book

[killianmuldoon]

User Story

As a developer I would like to quickly understand what I need to do to update my provider from v1alpha4 to v1beta1.

Detailed Description
The full list of changes and the requirements for providers to update their implementations for the new v1beta1 types should be documented in the CAPI book. These changes are already documented for previous changes to the API
a la https://cluster-api.sigs.k8s.io/developer/providers/v1alpha3-to-v1alpha4.html and the v1alpha4->v1beta1 documentation should be similar in form and content.

/kind feature

[killianmuldoon]

/kind documentation

[sbueringer]

/assign

[enxebre]

/milestone v1.0

[k8s-ci-robot]

@enxebre: You must be a member of the kubernetes-sigs/cluster-api-maintainers GitHub team to set the milestone. If you believe you should be able to issue the /milestone command, please contact your Cluster API Maintainers and have them propose you as an additional delegate for this responsibility.

In response to this:

/milestone v1.0

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