Add explanation of applicability in light of std::expected

[eyalroz]

C++23 added std::expected. It would be nice if the README for this repository would contain a section regarding whether, and how, the Outcome library is applicable when std::expected is available: Does it offer additional functionality? Alternative functionality with different design choices? etc.

(This could possibly also be combined with some info on the inside-boost and outside-of-boost versions and how they differ.)