[DOCS][IMPROVEMENTS] - Add Dependencies instructions for the "On-chain verification" tutorial #126

Hi, following your documentation, I noticed that it could be helpful to provide developers with the dependencies to install (in case they are not starting from your demo repo), especially the one for "@iden3" smart contracts.

The instruction to download the dependency is as follows: npm i @iden3/contracts.

Additionally, the iden3 package is not clearly indicated on their GitHub; but can be found it on npmjs or socket

Hope this feedback is constructive!

#ETHDenver2024