Doc: scope policies to a framework

The changes introduce a new resource, gitlab_group_security_policy_attachment, which allows attaching a security policy project to a group. It also adds a new field, policy_scope, to the gitlab_project_security_policy_attachment resource, enabling the attachment of multiple policies to a single group or project. The policies are provided as raw YAML, offering greater flexibility and support for various policy types and options.

This is coming from a customer who was not understanding how to scope a policy to a framework.