Add a introduction to Scala

[lefou]

With our latest ambitions to promote Mill as a (monorepo) build tool for languages other that Scala, our documentation should also target non-Scala developers. Some of them might even don't know Java, so some concept might feel strange to them and there might be some adoption barriers for Mill due to that (although nobody tells us about it).

I think it would be a good idea to either add a "Intro to Scala" section just complete enough to learn how to read a Mill build file or have some link collection that already provided that. After reading it, a developer should be able to understand

• what a object and a trait is and how they differ
• how functions and their return values are defined
• how import works
• that import $ivy is some non-Scala convenient way to load dependencies
• what package means
• That string interpolation exist and how it works
• probably more ...