* Fix dead link for `Rommel Marinez` Further Reading
Replace original link with archived version from wayback machine.
* Unify Further Reading References [nix/en]
Include live Susan Potter link that was previously fixed in `[nix/de-de]`.
- nix-repl now deprecated, Nix 2 has `nix repl`
- nix language now has floats, and type-preserving arithmetic
- add note about immutable sets' attributes being un-redefinable
- descendant sets can now have attributes added to them
- fix editing loose ene in the Impurity section
