Support cross-file go templates

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

Terraform: v1.11.3 on windows_amd64
tfplugindocs: latest (dev)

### Use Cases or Problem Statement

At the moment, it's only possible to reference and re-use different files via `codefile` and `tffile`. Both of them are markdown and it's not possible to include other template files or simply files without adding backticks for the markdown. The go templating function `template` can't be used because it doesn't get the references/defines of other files.
Adding this functionality would be very useful if several resources have common documentation parts like limitations or warnings.

### Proposal

Supporting the go templating function `template` so that it gets references and considers other template files.
e.g.,
```
templates/
├── partials/
│ ├── my-custom-template.tmpl
├── resources/
│ ├── my-resource.md.tmpl
```

`my-resource.md.tmpl`
```
# My Resource Documentation

This is the documentation for `my-resource`.

{{ template "my-custom-template" . }}
```

`my-custom-template.tmpl`
```
{{ define "my-custom-template" }}
## My Custom Template

This is reusable content for my custom template.
{{ end }}
```

### How much impact is this issue causing?

Low

### Additional Information

_No response_

### Code of Conduct

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