Document attaching multiple environments to the same policy

[dangrondahl]

Description

Within the environment policies documentation, include a section or example showing how to attach the same policy to multiple environments (e.g., staging and production sharing the same compliance requirements).

Proposed scope

• Example of a single policy attached to multiple environments via CLI and Terraform
• Use case: consistent compliance standards across staging/production
• Terraform example using kosli_policy_attachment with multiple environment resources referencing the same policy

Context

The kosli_policy_attachment Terraform resource and the kosli attach-policy CLI command already support this, but the docs should make the pattern explicit with practical examples.

Parent issue

Part of #167