🌱 Fix doc references to NodeCIDR in v1alpha8

[mdbooth]

Fixes: #1887

/hold

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[mdbooth]

@MaysaMacedo Please can you take a look at this, particularly the new doctext for Subnets?

[dulek]

/lgtm

[jichenjc]

/test pull-cluster-api-provider-openstack-test

[EmilienM]

/lgtm /hold cancel

[pierreprinetti]

Thank you @mdbooth !