[MAINT]: Document naming convention regarding plural and singular

### Describe the need

The general pattern has been to use plural on collections and singular on single resource operations.

For instance, repository collaborators ([single](https://github.com/integrations/terraform-provider-github/blob/main/github/resource_github_repository_collaborator.go), [collection](https://github.com/integrations/terraform-provider-github/blob/main/github/resource_github_repository_collaborators.go)) implements this approach.

We need to document in our contributor docs this pattern for future work.

### SDK Version

_No response_

### API Version

_No response_

### Relevant log output

```shell

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### Code of Conduct

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