Add package objects docs [tutorial] {EN}

[zafarella]

• sources:

• https://www.scala-lang.org/docu/files/packageobjects/packageobjects.html
• https://stackoverflow.com/questions/3400734/package-objects
• https://github.com/scala/scala/blob/2.13.x/src/library/scala/package.scala

[SethTisue]

thank you! this is a very nice addition to have.

[SethTisue]

I suggested some edits at #1236