Enhance templating functions

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

Terraform v1.13.4
tfplugindocs v0.25.0

### Use Cases or Problem Statement

Based on [this](https://github.com/hashicorp/terraform-plugin-docs/issues/591) and some use-cases I've stumbled upon (like that `$lastItem` range 😂 )

One major use-case: creating headings for Example files based on filename suffix

We're currently solving it with:
```
{{- range .ExampleFiles }}

{{- $parts := split . "resource_" -}}
{{- $lastItem := index $parts 1 }}
{{- $fileNameParts := split $lastItem "." }}
{{- $fileName := index $fileNameParts 0 }}

### {{ $fileName | title }}
```

but this is unwieldy and doesn't allow for replacing underscores in longer filenames

### Proposal

I would suggest adding https://github.com/masterminds/sprig functions to the template.

I'm willing to add a PR for this, if that would be fine with you :)

### 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

Start by locating where terraform-plugin-docs configures its Go templates and registers the existing functions; the issue does not name a file or test. Compare that setup with the proposed Sprig integration and verify the result against the documented ExampleFiles heading use case, including replacing underscores in filenames.

Written by the indexing model from the issue text.