Docs: mention Terraform Variables for phone call numbers

[case-fastly]

What is the improvement or update you wish to see?

As discussed in the Checkly community Slack channel, I'd asked about how to use the Phone Call alert channel without putting a phone number in the codebase.

The solution I ended up with, was using a Terraform Variable saved in Terraform Cloud.

Adding a snippet to the documentation suggesting this approach could help people in the future, for example:

  call {
    name   = "Phone call (Case)"
    number = "+12345678901" # or var.phone_number, to avoid storing a phone number in your codebase
  }

Is there any context that might help us understand?

None.

Does the docs page already exist? Please link to it.

No response