Update user documentation for 2.13 collections

[julienrf]

I create the issue here but the work will happen there: http://docs.scala-lang.org/overviews/collections/introduction.html

[julienrf]

Here is a list of sub-issues that should take a few days each:

• Collections guide: Update the collections guide to the new collections framework docs.scala-lang#995
• Migrating from 2.12: Add a documentation page about migrating from 2.12 to 2.13 docs.scala-lang#996
• Parallel collections: Update the parallel collections guide to the new version docs.scala-lang#997
• Architecture of collections: Port “The Architecture of Scala Collections” to the new architecture docs.scala-lang#960

[SethTisue]

"Document better the differences between views, iterators and lazylists", says scala/collection-strawman#466 (closing, consolidating here)

[NthPortal]

this section should be migrated to refer to and use LazyList

[julienrf]

@NthPortal There is a PR doing that: https://github.com/scala/docs.scala-lang/pull/1227/files#diff-d3d3d07b738099aae45ac84dda5abbefR21