Support marker-reference scope `volume`

[bertfrees]

This is supported in OBFL and in the Dotify API, but it is not implemented yet.

See brailleapps/dotify.formatter.impl#88.

The main use case is to to get the first and last print page in a volume, in order to print it on the title page.

As explained in brailleapps/dotify.formatter.impl#88 (comment), this use case would benefit from an additional change in OBFL. I will create a separate issue for that. (Edit: see mtmse/obfl#37.)