Instead of replacing the existing documentation, it creates a new
page for 2.13’s collections.

- Start by listing the challenges addressed by the collections
  framework,
- Fix the markdown syntax used for tables so that they can be rendered
  by GitHub,
- Mention that custom collections can be mutables or immutables,
- Add an appendix listing the transformation operations that are
  candidates to be overloaded to support the “same result type”
  principle,
- Add an appendix showing how to cross-build custom collection
  implementations with sbt.