Add FAQ for unused value of type cats.effect.IO[Unit] (add : Unit to discard silently)

[armanbilge]

This is the warning raised by the -Wnonunit-statement, at least on Scala 2 (someone should check Scala 3).

• Add -Wnonunit-statement to warn about discarded values in statement position scala/scala#9893
• Port -Wnonunit-statement setting for dotty scala/scala3#16936

We should:

1. document and recommend this very useful compiler option
2. get the error into our docs, so it is indexed and searchable for when folks hit it

[froth]

questions / thoughts before I invest time in this:

• should the recommendation the compiler option in getting-started? I would think so, because in FAQ it will get overlooked to easily.
• Adding something to FAQ additionally would be a good idea for more clarification and indexing the error
• Do we want to recommend/mention typelevel/sbt-tpolecat pending fix for -Wnonunit-statement seems not to work for scala-3 (3.3.3)  sbt-tpolecat#208 ?