make a tutorial on shared ownership (Rc, Arc, concurrent data structures)

[thestinger]

The coverage of managed pointers should be removed from the tutorial and perhaps replaced with a section mentioned shared ownership (in or after the ownership/boxes sections) and linking to the tutorial.

The shared ownership tutorial should explain when and why you would use shared ownership and the characteristics of the various available types.

[pnkfelix]

part of #11755

[steveklabnik]

This is a duplicate of #8395