Add note regarding minimal instance types for control plane nodes

[a13x5]

Some providers are having specific requirements for instance type (amount of resources) which is assigned to control plane nodes.

For example when using Azure Standard_B1s it will fail with the following message:

failureMessage: failed to reconcile AzureMachine service virtualmachine: failed to get desired parameters for resource my-azure-managedcluster4/my-azure-managedcluster4-cp-0 (service: virtualmachine): reconcile error that cannot be recovered occurred: VM size should be bigger or equal to at least 2 vCPUs. Object will not be requeued

We should add a note for all supported providers regarding a minimum that is required to create a cluster.

[bnallapeta]

Similar thing happens with OpenStack too:

[   14.473370] cloud-init[936]: [2024-11-13 12:12:30] [preflight] Running pre-flight checks
[   14.473970] cloud-init[936]: [2024-11-13 12:12:30] error execution phase preflight: [preflight] Some fatal errors occurred:
[   14.476860] cloud-init[936]: [2024-11-13 12:12:30] 	[ERROR NumCPU]: the number of available CPUs 1 is less than the required 2

And here are the docs for the same: https://cluster-api-openstack.sigs.k8s.io/clusteropenstack/configuration.html#machine-flavor