check: Verify import command resource type

[bflad]

This may require usage of a Markdown AST library, although unlike #21 we might be able to just simply strings.Contains("terraform import ") to focus on this line.

Import blocks should use the proper resource type (in example_thing resource page):

```console
$ terraform import example_thing.example ...
```

Its quite easy to copy-paste-forget this documentation.

[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.