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Document GHC 91999 and GHC 86639 #507

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Got two tickets at the workshop, wrote some quick docs - if there's anything that needs changing, please feel free to request.

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There is more info on deriving instances for extra typeclasses in the [GHC user
manual](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/deriving_extra.html).
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GitLab links can change, better to link documentation of a stable GHC release:

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There is more info on deriving instances for extra typeclasses in the [GHC user
manual](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/deriving_extra.html).
There is more info on deriving instances for extra typeclasses in the [GHC user manual](https://downloads.haskell.org/ghc/9.12.1/docs/users_guide/exts/deriving_extra.html).

`DerivingFunctor` language extension must be turned on. This can be done with a
pragma at the top of the file, or in build settings.

The `DerivingFunctor` extension is turned on automatically from Haskell2021
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The `DerivingFunctor` extension is turned on automatically from Haskell2021
The `DerivingFunctor` extension is turned on automatically from GHC2021


When deriving a `Functor` instance for a datatype in Haskell2010, the
`DerivingFunctor` language extension must be turned on. This can be done with a
pragma at the top of the file, or in build settings.
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It might be worth mentioning that DeriveTraversable implies DeriveFunctor and also fixes the error.

Any datatype that is exported from an .hs-boot file must be exported from the
corresponding .hs file.

More information on .hs-boot modules is available in [the GHC user manual](https://downloads.haskell.org/ghc/latest/docs/users_guide/separate_compilation.html#how-to-compile-mutually-recursive-modules).
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The anchor is gone, I guess it's now

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More information on .hs-boot modules is available in [the GHC user manual](https://downloads.haskell.org/ghc/latest/docs/users_guide/separate_compilation.html#how-to-compile-mutually-recursive-modules).
More information on .hs-boot modules is available in [the GHC user manual](https://downloads.haskell.org/ghc/9.12.1/docs/users_guide/separate_compilation.html#mutually-recursive-modules-and-hs-boot-files).

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