[testplanner] Restore the `foo_dv_plan.md` example
[testplanner] Restore the `foo_dv_plan.md` example

This file was removed unnecessarily during the hugo switch. Restore it
and update it to the new hugo style.

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan