fix: link built-in provider resources to the Terraform language docs - #955

Description of your changes

terraform_data and terraform_remote_state come from Terraform's built-in provider, which isn't published on the registry. In loadResources there is no required_providers entry to read a source from, so it falls back to synthesizing hashicorp/terraform (terraform/load.go:486-491), which is the deprecated external provider, and Resource.URL() then builds a registry link out of that. The generated link doesn't resolve.

This adds a carve-out in URL() that maps the built-in provider's resources to the Terraform language docs instead. terraform is a reserved provider local name and can't be declared in required_providers, so ProviderName == "terraform" is enough to identify the built-in provider unambiguously. An unrecognized built-in type returns an empty URL, which the resource templates already fall back to rendering as a plain spec.

Fixes #757

I have:

• Read and followed terraform-docs' contribution process.
• All tests pass when I run make test.

How has this code been tested

Added three cases to TestResourceURL: a built-in resource, a built-in data source, and an unrecognized built-in type.

make checkfmt, make test and make staticcheck all pass locally. make lint reports 9 issues, but they're all pre-existing: I get the identical 9 from a clean master worktree, and none of them are in the files this PR touches.

I also rendered a fixture module containing terraform_data, terraform_remote_state and tls_private_key.

Before:

After:

The tls_private_key row is unchanged.