Missing template functions

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

- `Terraform v1.14.0 on linux_amd64`
- `tfplugindocs Version 0.24.0 from commit v0.24.0`

### Use Cases or Problem Statement

I'm using custom templates and would need some `hasSuffix` function for strings in it.

Currently I have to solve this via the split function which is pretty ugly and not straight forward.

https://github.com/stackitcloud/terraform-provider-stackit/blob/5e12fb90e44636e388cebaa68fb42eb39487c0d5/templates/resources.md.tmpl#L8-L21

### Proposal

**Option 1: Add more template functions to the existing list**

Add https://pkg.go.dev/strings#HasSuffix to the list of the template functions:

https://github.com/hashicorp/terraform-plugin-docs/blob/e5b0eeb7818b2baab5c8c0340790b4a936406c4c/internal/provider/template.go#L113-L123

**Option 2: Allow users to register their own template functions**

Not sure if that's easy to implement.

### How much impact is this issue causing?

Low

### Additional Information

Will create some PR for Option 1 later.

### Code of Conduct

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