[Suggestion]: add "Part 3" to the Overview of Ray

[kamil-kaczmarek]

Please share your suggestion here

Add Part 3, that will consist of small coding exercises:

Work with Object store

• add object with ray.put()
• print returned object reference
• use ray.get() to access value of the object.
• Mention that tasks and actors return futures that are references as well.

Compute pi digits
Use this docs example to show highly_parallel computational job - compute pi digits.