Be more explicit about "magic" in docs examples

By "magic" I mean imports, implicits, etc. By explicit, I mean that e.g. even if none are necessary, maybe call that out. If they are canonical, say that too. "This is the one-and-only and therefore is the right one for you " :)

Quoting @davesmith00000:

Most of the cats-effect docs do not show imports or how the implicit typeclass instances are provided. I know how to do that, I don't know what you're recommending, nor do I know if I've got the right import from the right package. There may only be one choice in fact, it's just not very... reassuring. When I'm pulling my hair out at 3am, reassurance is nice.

https://discord.com/channels/632277896739946517/632278585700384799/959431282088103987