Migration recipe from DRL to CS

As a blog post, linked from the upgrade recipe.

• Intro: some motivation (why would I want to migrate).
  • Maybe a link to the old CS blog post that lists advantages in depth? https://www.optaplanner.org/blog/2020/04/07/ConstraintStreams.html
• Main piece: before and after code of common code patterns, such as:
  • conflict constraints such as "room conflict"
  • capacity limitation constraints such as "vehicle capacity" or "employee working hours per week limitation"
  • skill constraint
  • fairness
  • consecutive shifts
• Outro