feat: API improvements

[gavriliumircea]

This pr aims to implement some improvements for the merkle api. Bellow you can find a list with the improvements:

• add example for csv document format inside README (#17 )
• add pinata access control token and fix #8
• enforce consistency for error responses on all environments #7
• create some unit tests(address #3)

[PaulRBerg]

I will review this during the weekend, but if you urgently need it on main feel free to merge and I will create another PR

[PaulRBerg]

@gavriliumircea feel free to merge this once you review the comments above