Move documentation of deriving of `Ix` instances into the report

Currently, the specification of derived instances of the `Eq, Ord, Bounded, Enum, Show, Read` typeclasses lives in a chapter in the language report, whereas the documentation of derived instances of the `Ix` typeclass is part of the library report.

This is partly justified by the fact that `Ix` is not part of the Prelude, but otherwise there does not seem to be a good reason for this organization choice. Since we probably want to specify extensions such as `DeriveFunctor`, `DeriveFoldable` etc. in future versions of the report, I think it would make more sense to document all `stock` deriving strategies in one place in the report.

Locate the language-report chapter covering derived Eq, Ord, Bounded, Enum, Show, and Read instances, then find the current Ix deriving documentation in the library report. Move the Ix specification into the language report alongside the other stock deriving strategies; done when the Ix documentation is no longer maintained in the library report and the report remains coherent.

Written by the indexing model from the issue text.