feat: add video series on contract security (#2425)

Co-authored-by: Guillermo Alejandro Gallardo Diez <[email protected]>

docs/2.build/2.smart-contracts/security/welcome.md

@@ -20,10 +20,16 @@ Make sure to keep your smart contract safe by running through our [security chec
 ## 🛡️ Security Concepts
 Learn important security concepts to keep your dApp safe and sound.  Read about:
 
-- Keeping [callbacks safe](./callbacks.md).
-- Understanding [frontrunning](./frontrunning.md).
-- Understanding [sybil attacks](./sybil.md).
-- Understanding [reentrancy attacks](./reentrancy.md).
-- Making sure the owner of an account is [the one calling](./one_yocto.md).
-- Understanding the complexity of [generating random numbers](./random.md).
-- Protecting from a [small deposit attacks](./storage.md) draining your account.
+- Keeping [callbacks safe](./callbacks.md)
+- Understanding [frontrunning](./frontrunning.md)
+- Understanding [sybil attacks](./sybil.md)
+- Understanding [reentrancy attacks](./reentrancy.md)
+- Making sure the owner of an account is [the one calling](./one_yocto.md)
+- Understanding the complexity of [generating random numbers](./random.md)
+- Protecting from a [small deposit attacks](./storage.md) draining your account
+
+## 🎞️ External Resources
+
+An awesome video series on Smart Contract Security by [https://twitter.com/timurguvenkaya](https://twitter.com/timurguvenkaya) can be found on YouTube:
+
+<iframe width="100%" height="315" src="https://www.youtube.com/embed/DrRr6nru0no?si=FxLJK-LYHrAqX0hc" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>