periodic: Move 320.whatis to the mandoc package

This periodic script only makes sense if mandoc is installed, so move
it to the mandoc package like other periodic scripts.

/usr/libexec/makewhatis.local only exists for the enjoyment of this
script, and doesn't work without mandoc installed, so move that as
well.

This change moves files between packages so, until we have a proper
policy on how to handle this in release/stable branches, it should
not be MFC'd.

MFC after: never
Reviewed by: ziaee, manu
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53609