Deprecate the terraform-provider-metal project

To consider it fully deprecated all boxes below must be checked (same for original issue created in terraform-provider-packet):

[x] Migration guide should be prepared https://github.com/equinix/terraform-provider-equinix/blob/master/docs/guides/migration_guide_equinix_metal.md
[x] #235 README.md should be updated with a deprecation notice (refer to github.com/packethost/standards)
[x] GitHub project description and link should refer to the new provider
[x] Issues should be migrated to the new project (or closed and reopened if valid)
[x] Terraform Registry should be updated with a deprecation notice (index.md)
[x] Terraform resource documentation should be updated with a deprecation notice in support of the new resources
[x] Terraform provider should include any deprecation messages available through metadata or warning messages (https://www.terraform.io/plugin/sdkv2/best-practices/deprecations#example-resource-renaming is this available in v1 sdk?)

equinix / terraform-provider-metal