Document (point to documentation) on how to run Terraform from Terraform CLI

[glennmusa]

Benefit/Result/Outcome

So that a user doesn't need to "know MLZ" to deploy the Terraform template in the repository, especially in a future where MLZ doesn't ship BASH scripts to do it on the user's behalf.

Description

In a future where we don't rely on the MLZ BASH scripts to deploy Terraform, but instead, defer that responsibility to the native Terraform tooling, we need to point the user to Terraform CLI docs on how Terraform apply and destroy work, how to find the appropriate value for the tf_environment variable, and how to use their own credential or a service principal credential.

Acceptance Criteria

• Point to documentation on how to terraform apply and terraform destroy
• Point to documentation on which Terraform tf_environment value to use
• Point to documentation on how to run terraform as a Service Principal

[brooke-hamilton]

Related to #355