Add relative-distance exercise

[kahgoh]

The relative distance exercise was recently added to the problem-specs.

[Caskader]

It is given that a person and his father's brother will have a separation of 2 but in the example it is said that Uma and Tariq have a separation of 3.

#given_instructions

[jagdish-15]

This exercise has already been added to the track by @BNAndras
reference: #2947
Let's close this!

[kahgoh]

Thanks for mentioning that @jagdish-15! I agree that we can close this because the exercise has been added.