Rework Terraform plugin for PR flow

Per multiple feedback, we should rework the Terraform plugin flow to mimic the collaborative _Terraform Cloud_ model which fits better production use of Terraform in the wild.

The `Terraform` plugin should integrate with Github and Gitlab to be able to work nicely with the PR/review/approve flow where the `terraform plan` diff is included in the PR comment via bot which the reviewer can approve.
Additionally, we should make sure to include sufficient docs and examples to guide users with the best practice.

### Resources
- https://cloud.hashicorp.com/products/terraform
- Example: https://github.com/marketplace/actions/terraform-pr-commenter
- Screenshots: https://github.com/marketplace/actions/terraform-pr-commenter#screenshots

Start by reviewing the Terraform plugin and the linked Terraform Cloud and terraform-pr-commenter resources. Map the requested GitHub and GitLab pull-request flow, plan-diff comments, approval behavior, and documentation/examples; done means these requested integrations and guidance are implemented.

Written by the indexing model from the issue text.