Document Key and mention it in the tutorial

[nh2]

In https://hackage.haskell.org/package/aeson-2.1.1.0/docs/Data-Aeson-Key.html#t:Key there are no haddocks, and the "How to use this library" section has not a single example that shows non-literal keys being used (coming e.g. from Text).

It would be great if:

• Haddocks were added to Key that motivate its existence, so that e.g. hovering it from an IDE gives tips on how to obtain that type
• an example of keys being converted from Text, so that beginners or upgraders from pre-2.0 have an esier time

[nh2]

A bit more detail:

If you read the docs of (.:) and click on Key in the type signature, the page jumps to a re-export of Key in the same module where Key is an opaque type without any information on how to make one.

[phadej]

the page jumps to

That's unfortunate. I wonder what happens if we change the haddock link to 'Data.Aeson.Key.Key', so it would jump to correct module?