Documentation: Un-hardcode the Scala version used
Documentation: Un-hardcode the Scala version used

Do not mention the Scala version used in the document, as it is already
solely specified through the build.sbt file; remove that duplication.
Therefore merge the Prerequisites (Scala) and How-to-build sections.

Clarify that sbt is expected in order to build using that Scala version.

Change-Id: I94c788ede8e793073199dc3a96e782fd6ef74f20

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