Detailed Exit Codes

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

`github.com/hashicorp/terraform-plugin-docs v0.19.3`

### Use Cases or Problem Statement

Currently, when the `tfplugindocs` commands are executed there are no exit codes to explicitly indicate whether the command run was successful or not. While this may be fine for the `tfplugindocs migrate` and `validate` commands, I believe having a detailed exit code that indicates what exactly happened would be a good way to assess changes, as opposed to having to look at each `.md` file manually. These codes will indicate to the contributor to be easily aware of the status of their changes as it relates to the tool.

### Proposal

Some detailed exit codes:
-detailed-exitcode Return detailed exit codes when the command exits.

This will change the meaning of exit codes to:

0 - Succeeded, diff is empty (no changes)

1 - Errored

2 - Succeeded, there is a diff

### 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