Adding Documentation for Data/MC Plot

[afroch]

Hmm maybe I should have asked to update this docs here before merging. Can you do this in a follow up MR @afroch?

Originally posted by @samvanstroud in https://github.com/umami-hep/puma/issues/187#issuecomment-1602239299

[samvanstroud]

Friendly ping @afroch on this one