check: Verify code block types

[bflad]

Reference: https://github.com/terraform-providers/terraform-provider-aws/pull/11302

This likely will require usage of a Markdown AST library.

Example usage blocks should use:

```terraform
# ... Terraform configuration ...
```

Import blocks should use:

```console
$ terraform import ...
```

[bflad]

Experimental support for this has been added via new -enable-contents-check flag in version 0.8.0 of tfproviderdocs.

[bflad]

This project will be archived in preference of the revamped validate command in github.com/hashicorp/terraform-plugin-docs implementing much of the basic validation logic from this project. Feature requests should be submitted over there at this point.