Make use of pre-commit's ability to run golang tools natively for terraform-docs

#13 requires that terraform-docs be run in a way amenable for the renovate update environment. In User Notify and the general GCP Boilerplate we've done that by having terraform-docs be run using pre-commit's native golang tool support.

We can't use the same approach for tflint at the moment since that uses a custom plugin which is non-trivial to install via pre-commit.