Issue 12783: Added 'Third Party Libraries' links to the sidebars

Andrei was talking about code.dlang.org not being particularly well known at DConf today. I had actually never heard of it prior to then. I think that this problem is one that is best solved by doing something quite simple: Adding a link in the sidebars to point to this wonderful collection.

https://issues.dlang.org/show_bug.cgi?id=12783