nixos/hedgedoc: remove minio from examples

        Nix store, by specifying placeholder variables as the option value in Nix and

        setting these variables accordingly in the environment file.

        Snippet of HedgeDoc config containing a secret:

        ```

          # snippet of HedgeDoc-related config

        services.hedgedoc.settings.dbURL = "postgres://hedgedoc:\''${DB_PASSWORD}@db-host:5432/hedgedocdb";

          services.hedgedoc.settings.minio.secretKey = "$MINIO_SECRET_KEY";

        ```

        ```

          # content of the environment file

        and the content of this environment file:

        ````

          DB_PASSWORD=verysecretdbpassword

          MINIO_SECRET_KEY=verysecretminiokey

        ```

        Note that this file needs to be available on the host on which

        `HedgeDoc` is running.

      '';

    };

  };