Update config.yaml annotation with carpentry_description option

When https://github.com/carpentries/sandpaper/pull/585 is merged, the template `config.yaml` here should be updated to include the comments documenting the `carpentry_description` option.

First verify that carpentries/sandpaper#585 has merged, then open the template config.yaml file. Compare its existing annotations with the comments introduced by that change and update the template to document carpentry_description; the work is done when the option is documented consistently in the template.

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