docs: add a note about the default clusterclass and how to get it

[faiq]

No description provided.

[jimmidyson]

The clusterctl generate command below actually creates this ClusterClass so unless you want to create it manually for other reasons this step is unnecessary.

$ clusterctl generate cluster capi-quickstart --flavor development --kubernetes-version v1.27.2 --control-plane-machine-count=1 --worker-machine-count=1 | gojq --yaml-input --yaml-output '. | select(.kind=="ClusterClass")'

apiVersion: cluster.x-k8s.io/v1beta1
kind: ClusterClass
metadata:
  name: quick-start
  namespace: default
<SNIP>

[jimmidyson]

Ah I see what the problem is, the kubectl label messes up the YAML. Tested the following and it works - I'll push a new commit with this change to this PR (can always revert if you prefer to keep it this way):

clusterctl generate cluster capi-quickstart \
    --flavor development \
    --kubernetes-version v1.27.2 \
    --control-plane-machine-count=1 \
    --worker-machine-count=1 | \
  gojq --yaml-input --yaml-output \
    '. | (select(.kind=="Cluster").metadata.labels["capiext.labs.d2iq.io/cni"]|="calico")' | \
  kubectl apply --server-side -f -

[jimmidyson]

On second thoughts, I'm going to merge #107 and could you test again please? I think that should have everything you need to get going without this change.