Update index.md

Adding Cyfrin Updraft (https://updraft.cyfrin.io) as an educational resource to learn Solidity and smart contract development.

Cyfrin is the smart contract security firm behind Updraft - the ultimate educational platform for smart contract development and auditing. Leveraging over 100+ hours of content from industry experts, Patrick Collins is the main teacher behind the platform. With hundreds of thousands of views in his courses, Updraft's content teaches everything from Solidity basics all the way to Advanced Foundry, Hardhat, Security, DeFi, and more - all containing hours of content on Ethereum development and auditing.

Author: juliettech13

src/content/developers/docs/smart-contracts/index.md

@@ -109,3 +109,4 @@ Multisig (multiple-signature) contracts are smart contract accounts that require
 - [Coinbase: What is a smart contract?](https://www.coinbase.com/learn/crypto-basics/what-is-a-smart-contract)
 - [Chainlink: What is a smart contract?](https://chain.link/education/smart-contracts)
 - [Video: Simply Explained - Smart Contracts](https://youtu.be/ZE2HxTmxfrI)
+- [Cyfrin Updraft: Web3 learning and auditing platform](https://updraft.cyfrin.io)