Allow setting subcategory from code

Hi,

per the [docs](https://www.terraform.io/registry/providers/docs#subcategories), subcategories can be used to add some structure to the docs.
The default template defaults to `""`, which I could then manually adjust.

The problem with this is this: I maintain a provider with many external contributions are via pull requests. We have a pipeline that enforces that the documentation is updated by running `tfplugindocs` on the PR and then erroring if there are any changes. This means the author of the PR themselves must run `tfplugindocs` if they introduce a new attribute or resource to update the docs in their PR.

This means managing the subcategory manually is not an option.