Add Maven and Amper configurations/examples in the documentation

[koperagen]

Coroutines have configuration for Maven
https://github.com/Kotlin/kotlinx.coroutines

As i said here, for Maven we don't have a plugin now and we can mention it
#717 (reply in thread)

But for Amper it should be possible to use Gradle plugin too
Gradle interop, including combining Amper and Gradle modules in one project*.

[Jolanrensen]

I'd like to bump up this issue. Especially since the compiler plugin now works regardless of the build system used!

Showing off Amper would be good for dogfooding and showing Maven would be good for JVM ecosystem integration, so both have benefits! Not everyone uses Gradle, after all ;P