Sentinel example needs clarification in README

[lisamurphy-msft]

Description

The Terraform example for deploying Sentinel does not make it clear that the purpose of the example is to show how to deploy a Terraform template after MLZ has been deployed using the Bicep template.

Expected Behavior

The first paragraph of the README should tell the reader that the template demonstrates how to deploy a Terraform template into a Mission Landing Zone instance that has been deployed using Bicep.

Actual Behavior

The README says that it's a Terraform template, but does not say why the example is useful.

[brooke-hamilton]

@lisamurphy-msft I updated the description of the bug to add expected and actual behavior. We don't need an additional Bicep implementation of Sentinel (the core MLZ deployment already deploys Sentinel)--we need to tell readers why this example is here as Terraform deploying into an MLZ environment that was provisioned using Bicep.

I also updated the bug's title and removed the enhancement tag.