doc: try to use groff if man2html is not found
doc: try to use groff if man2html is not found

to generate a html version of the manpage,
throw a fatal error otherwise.

afh/ledger,paulbdavis/ledger,ledger/ledger,afh/ledger,jwakely/ledger,ledger/ledger,paulbdavis/ledger,duncanmortimer/ledger,paulbdavis/ledger,jwakely/ledger,jwakely/ledger,duncanmortimer/ledger,ledger/ledger,duncanmortimer/ledger,afh/ledger,paulbdavis/ledger,ledger/ledger,jwakely/ledger,duncanmortimer/ledger,afh/ledger,afh/ledger,paulbdavis/ledger,paulbdavis/ledger,jwakely/ledger,duncanmortimer/ledger,duncanmortimer/ledger,ledger/ledger,paulbdavis/ledger,duncanmortimer/ledger,jwakely/ledger,jwakely/ledger