check: Implement Example Usage header check

[bflad]

Terraform Provider resource/data source documentation pages should include an example usage section.

Failing case:

---
... Frontmatter ...
---

# Title

Byline

## Argument Reference

## Attribute Reference

Passing case:

---
... Frontmatter ...
---

# Title

Byline

## Example Usage

## Argument Reference

## Attribute Reference

[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.