Add supply chain attacks to "Common Threats" page

I think it could have its own section and have been working on a PR in this regard.

It’s an important issue, especially with software these days having a lot of external dependencies.

We see some articles about it for modern tooling:

We also see some examples of that in:

One of the most famous ones in the past was the patches that Debian applied to openssl random number generator CVE-2008-0166. I’m not sure whether malicious intent was ever proven like with the XZ vulnerability eg (purposefully obfuscated code).