📖 add disclaimer about clusterctl topology dryrun RuntimeSDK support

[sbueringer]

What this PR does / why we need it:
Adds a disclaimer that clusterctl topology dryrun doesn't support RuntimeSDK yet.

When using the command with a ClusterClass with external patching the following error is produced:

➜ ${CAPI_HOME}/bin/clusterctl alpha topology plan -f docker-cluster.yaml -f docker-clusterclass.yaml --output-directory=/tmp/test
Detected a cluster with Cluster API installed. Will use it to fetch missing objects.
Error: failed defaulting and validation on input objects: failed to run defaulting and validation on ClusterClasses: failed validation of cluster.x-k8s.io/v1beta1, Kind=ClusterClass default/quick-start: ClusterClass.cluster.x-k8s.io "quick-start" is invalid: spec.patches[0].external: Forbidden: patch.external can be used only if the RuntimeSDK feature flag is enabled

I can change/improve that but it would require ~ something like this: https://github.com/kubernetes-sigs/cluster-api/compare/main...sbueringer:cluster-api:poc-clusterctl-runtimesdk?expand=1

Please note adding the disclaimer is just a stop gap solution until we got around to implementing it correctly in clusterctl (see #6545)

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Part of #6330

[sbueringer]

/cc @fabriziopandini @ykakarap

[killianmuldoon]

/lgtm

[ykakarap]

/lgtm

[ykakarap]

How about making the SSA support and the RuntimeSDK points as separate bullet points? But keep both the points in the Limitations block.

[sbueringer]

How about making the SSA support and the RuntimeSDK points as separate bullet points? But keep both the points in the Limitations block.

Not sure the SSA section is 3 separate sections (+ one of them contains a listing). I would probably prefer making it a separate note if we want to split it up

[ykakarap]

Not sure the SSA section is 3 separate sections (+ one of them contains a listing). I would probably prefer making it a separate note if we want to split it up

+1 to splitting it up.

[ykakarap]

/lgtm

[vincepri]

/approve

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[sbueringer]

/cherry-pick release-1.2

[k8s-infra-cherrypick-robot]

@sbueringer: new pull request created: #6852

In response to this:

/cherry-pick release-1.2

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