Update Documentation for TEM Domain Resource to Specify Region Limitation

[jremy42]

Title

Update Documentation for TEM Domain Resource to Specify Region Limitation

Description

The current Terraform documentation for the tem_domain resource does not explicitly specify that only the fr-par region is supported. This leads to user confusion, as:

1. The API and Terraform provider accept a region argument.
2. The UI does not allow selecting a region.
3. Attempting to create a tem_domain resource in a region other than fr-par (e.g., nl-ams) results in a 501 "Not Implemented" error.

Steps to Reproduce

1. Use the Terraform provider to define a tem_domain resource with the region set to anything other than fr-par.
2. Run terraform apply.
3. Observe the process waiting for resource creation and eventually failing with a 501 error.

Proposed Solution

1. Update the Terraform documentation to explicitly state that the region argument must be set to fr-par for the tem_domain resource.
2. Add a warning or note in the documentation to inform users that TEM is a global product and the region argument is included only for API consistency.
3. (Optional) Update the Terraform provider to validate the region argument and throw a descriptive error if a region other than fr-par is provided.

Additional Context

Users have reported frustration with the current behavior, as described in [this Slack thread](https://scaleway-community.slack.com/archives/C0358JE0XEK/p1738011678941619).