[docs] correct enumeration of "key contracts"

[pdg744]

the enumeration of key contracts in the contracts readme is self-contradictory -- it says "the 2 key contracts you'll need [as a user]..." and then goes on to enumerate 4 contracts (only some of which you need to think about as a user)

[pdg744]

My instinct is that the right approach here is to have the top-of-page focus on what users need to think about and move the implementation details to a separate section.