KubeOne: Digital Ocean Projects not documented

Digital Ocean now have so-called *projects* which resources can be assigned to.

- https://docs.digitalocean.com/reference/terraform/reference/resources/project/

This allows creation of resources under different name spaces.

Its usage is currently not documented for KubeOne.

Start by reading the linked DigitalOcean Terraform project resource reference and locating the KubeOne documentation for DigitalOcean resources. Document how DigitalOcean projects can be used with KubeOne, including the relevant resource-assignment usage, and verify that the resulting documentation is clear and complete.

Written by the indexing model from the issue text.