Update the parallel collections guide to the new version

[julienrf]

http://docs.scala-lang.org/overviews/parallel-collections/overview.html

We should probably move the parallel collections documentation away from docs.scala-lang.org, like we did with scala/scala-xml.

[SethTisue]

the main things I can think of that could/should change would be to

• add the information that as of 2.13 it's a separate dependency
• make any needed updates now that parallel collections have their own inheritance hierarchy, instead of being part of the standard collections hierarchy