Run terraform docs pre-commit hook automatically when terraform providers are updated

Using the same approach as in https://gitlab.developers.cam.ac.uk/uis/devops/user-notify/infrastructure/-/issues/37, update the template to run terraform docs whenever terraform providers and/or the template itself is updated by renovate.

We probably want to wait on uis/devops/renovate-config#31 (closed) which adds a generic "run pre-commit hook" preset to renovate-config.