doc: allow redirect old url for nrf9160 DK documentation page

We are renaming the nrf9160 DK documentation page, and due to
re-organizing the doc directory we need to allow redirecting
the old URLs for nrf916 DK board targets' documentation.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>