doc: mention new documentation-reviewers subteam

To get more information about how to review specific parts of Nixpkgs, refer to the documents linked to in the [overview section][overview].

If a pull request contains documentation changes that might require feedback from the documentation team, ping @NixOS/documentation-team on the pull request.

If a pull request contains documentation changes that might require feedback from the documentation team, ping [@NixOS/documentation-reviewers](https://github.com/orgs/nixos/teams/documentation-reviewers) on the pull request.

If you consider having enough knowledge and experience in a topic and would like to be a long-term reviewer for related submissions, please contact the current reviewers for that topic. They will give you information about the reviewing process. The main reviewers for a topic can be hard to find as there is no list, but checking past pull requests to see who reviewed or git-blaming the code to see who committed to that topic can give some hints.

      _Default value:_ the output path's hash.

  ```

## Getting help

If you need documentation-specific help or reviews, ping [@NixOS/documentation-reviewers](https://github.com/orgs/nixos/teams/documentation-reviewers) on your pull request.