digital_electricity: tutorial: NOR gate added

Basic tutorial on NOR gate added, with the goal of lighting the digital
light using a NOR gate placed in the playArea and input lists consisting
of "one" input and NOT gate. This level is aimed at teaching the basics
of the NOR gate to the user.

Signed-off-by: Rudra Nil Basu <rudra.nil.basu.1996@gmail.com>