📖 Document new scheme-builder patterns in provider migration doc

[furkatgofurov7]

What this PR does / why we need it:
Documents changes from #9045 in v1.5-v1.6 migration document

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
xref comment: #9045 (comment)

[furkatgofurov7]

/assign @vincepri

/cc @sbueringer @JoelSpeed

[JoelSpeed]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: c91da8c98c8401b46882a5d018d13604c286284a

[killianmuldoon]

Instead of replicating the info here could this just be a short note and a link to where this is fully documented?

e.g. https://main.cluster-api.sigs.k8s.io/developer/providers/implementers-guide/create_api#registering-apis-in-the-scheme

[furkatgofurov7]

Instead of replicating the info here could this just be a short note and a link to where this is fully documented?

e.g. https://main.cluster-api.sigs.k8s.io/developer/providers/implementers-guide/create_api#registering-apis-in-the-scheme

@killianmuldoon I am fine with this suggestion as well. Do I understand correctly, in that case I can also drop the diff go code changes that I currently have or leave it?

[killianmuldoon]

I've added a note with a suggestion of how I think this should look - please check the link though as I didn't verify it.

[killianmuldoon]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: bed23d548e9cb9414e53fc27d99887e2296704ba

[sbueringer]

/lgtm
/approve
Thx!

[k8s-ci-robot]

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