Refactor computed properties chapter

[NataliaTepluhina]

From #31 (comment) and #31 (comment)

1. An example with reversedMessage on Computed properties chapter is introducing to complex handling of strings which is out of the scope here. We can add a simpler example something like the sum of two numbers as a computed property.

2. The example about in-template expressions makes sense, but we introduce computed properties as a mechanism to get derived values. The introduction as a solution to prevent complex expressions in templates does not sound that impressive.