Document subnet_id

[process0]

The ability to specify a subnet_id exists but there is no documentation about how to retrieve this ID. The hcloud tool does not list this identifier in the hcloud network describe either. After creating a subnet in terraform and then outputting it's ID, it was clear that this is created from network_id-subnet_range, i.e. 123456-10.0.0.3/24. Is this a format we can depend on not changing?

[process0]

Related https://github.com/hetznercloud/terraform-provider-hcloud/issues/340 that subnet_id is essentially a hack to ensure the subnet exists within the network.

[github-actions[bot]]

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