Allow subcategories under the "Guides" heading

### Terraform CLI and terraform-plugin-docs Versions

any version

### Use Cases or Problem Statement

The TF registry docs do not allow subcategories under the "Guides" heading and suggests breaking out additional guides into new headings ([docs](https://developer.hashicorp.com/terraform/registry/providers/docs#guides-subcategories)). This makes it hard for the user to figure out what is a Guide or not.

### Proposal

We want the same ability we have for other parts of the documentation, such as Resources and Data Sources, to better use subcategories for user discoverability.

### How much impact is this issue causing?

Medium

### Additional Information

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct