Add a way to do "explain" ala ArchiveBot

[Sanqui]

There are three ways I can think of implementing this:

1. Add an explain for jobs
2. Add an explain for individual tasks
3. Add an "Explain" task (as suggested in https://github.com/Sanqui/discard2/issues/6#issuecomment-1151574089)

1. and 2. aren't mutually exclusive and I think are good ideas. The job explain could be simply provided using --explain on the command line. I can't think a good reason for 3. but maybe @TheTechRobo can give some example use case?

[TheTechRobo]

I can't think a good reason for 3. but maybe @TheTechRobo can give some example use case?

It was more an implementation idea for 1.