Added some examples of `parEvalMap` to the *Concurrency* section in the documentation guide.

Added reasoning about `merge` halting variants and an example of `concurrently` using a producer consumer demo (this example only works in Scala 3)

Added a similar consumer-producer example to the `parJoin` method.
Also mentioned the `parJoinUnbounded` version.

Apparently, `mdoc` does not have a way to distinguish between Scala 2 and Scala 3 AFAIK. So this commit changes the examples in Scala 3 to Scala 2.

The sample routes in the example are actually linked to the project, so I used the testdata/ folder to place some lorem ipsum files.

The previous one had conflict because of previous definitions in the `concurrently` one.

The new example also runs three streams at the same time to differentiate it a bit more from `merge`.

Changed tree 🌳 to three in typo on `merge`

Co-authored-by: Michael Pilquist <mpilquist@gmail.com>

Changed 'optimizations' word from british to american english

Co-authored-by: Michael Pilquist <mpilquist@gmail.com>

Removed unused imports in various examples before `mdoc:reset`

Co-authored-by: Michael Pilquist <mpilquist@gmail.com>

Changed the example as suggested.

Now it does not parse the bytes, nor reads from the filesystem. Instead it downloads some files from The Project Gutenberg and creates an `InputStream` from each one of them.

Co-authored-by: Michael Pilquist <mpilquist@gmail.com>