Document `Key` and mention it in the tutorial

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](https://hackage.haskell.org/package/aeson-2.1.1.0/docs/Data-Aeson.html#g:1)" 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