Improve wordings in actor-systems.md
Adding Java samples in actor-systems.md, and description for scala.concurrent.blocking() inside Future
Move the section for blocking operations to dispatchers.md
Fix minor issues in dispatchers.md
Remove note about scala.concurrent.blocking which would be unnecessary.
Correcting a typo "run run"
* Identical contents in scala/cluster-metrics.md and java/cluster-metrics.md for easier review
* Replace java/cluster-metrics.md with a symlink to scala/cluster-metrics.md
* Fix typo
* Java version of new Hello World
* Define paradox groups, use shorthand directives
* Make tutorial apply to both java and scala
* Use sbt-paradox-akka
