Add Terraform module types explainer and Best Practices reference

Following the recent approval of the Tech Lead Forum issue to distinguish between two distinct types of Terraform module, this issue aims to document the proposal in the guidebook.

This should be presented as an explainer section, linked to a new Terraform Best Practices reference page. Over time, we plan to add more links to this reference page, creating a central resource where new starters and others can easily find our recommended Terraform best practices.

Acceptance criteria

• New Terraform best practices page is created at /reference/best-practice/terraform.
• New Terraform module type explainer is created at /explanations/terraform/module-types.
• Chore: move the existing /explanations/cloud-platform/testing-terraform-modules page to the /explanations/terraform section.