The smart_pointers section is too big

When following the exercises, it is nice to check the suggested reading.
For the smart_pointers exercises, it includes a lot of reading:

• Smart Pointers
  -> the full chapter about Smart Pointers in The Book. It is a big chapter.
• Using Box to Point to Data on the Heap
  -> Repetition. This is included in the Smart Pointers chapter
• Rc<T>, the Reference Counted Smart Pointer
  -> Repetition. This is included in the Smart Pointers chapter
• Shared-State Concurrency
  -> This is included in the chapter Fearless Concurrency in The Book.
  To understand it, it is better to read the full chapter, and it is a Big chapter.
• Cow Documentation
  -> Advanced topic about Copy-On-Write (actually, Clone-On-Write in Rust), a performance optimization technique, which requires to check other sources (example) to understand well. (This is not actually covered in The Book).

The exercises and reading to do before the Smart Pointers section are quite fast to get through. However, reading the references for 2 full chapters in The Book + researching about the Cow type is more time consuming. So, less engaging.

I would suggest to move the arc1 exercise under the threads topics to not have to read about concurrency yet.

Rustlings v5.6.1

Review the smart_pointers exercise references, especially the Smart Pointers and Fearless Concurrency chapters and the Cow documentation linked in the issue. Check how the arc1 exercise is grouped with the threads topics. Done means the reading is less repetitive and appropriately scoped, with arc1 moved if that change is accepted.