📝 Add more examples / uses to the Concurrency section in the documentation guide of Fs2

[Hombre-x]

Helps with #1958

What's included:

• Examples for both parEvalMap and parEvalMapUnordered
• Reasoning about the three merge variants; it's about when they will halt or not depending on the provided streams
• Added a concurrently example using a simple producer-consumer environment (Had to use Scala 2 syntax with implicits, I didn't know how to make mdoc work with Scala 3 at the same time 🤔).
• Changed the previous example to make it suitable for parJoin