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).

• https://github.com/scala/scala/pull/9893
• https://github.com/lampepfl/dotty/pull/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 https://github.com/typelevel/sbt-tpolecat/issues/208 ?