Make output directory defaults and docs consistent

The staticvolt tool used '_site'. The StaticVolt.pm code and docs used '_destination'.

This confused me for longer than it should have done when first using StaticVolt :(

This change makes the default everywhere '_site', as I'm guessing that would be your preferred location.

There is not any sort of CHANGES file to update, but I guess it should be mentioned in any future release notes.