Improve documentation of JSON argument source

[nipafx]

Two things:

• reduce code snippet line length to max ~60, so they fit into the narrow code boxes on the website (i.e. no horizontal scroll bar)
• explain how parser integration works:
  • so far only Jackson
  • it's an optional dependency, so if you want to use the extension, you need to do $THING_A in Gradle (feature variant) and $THING_B in Maven (just add dependency, I think) - see https://github.com/junit-pioneer/junit-pioneer-demo/tree/jackson-feature (and update/merge that branch into main while at it)
  • if you're using modules but have no dependency on Jackson, add requires ...jackson to module descriptor or --add-module to test runtime so it makes it into module graph
• mention that text blocks work

[Bukama]

Max line length is 60 (see #317)

[nipafx]

@Bukama Thanks, updated the description.