The "derivation" is one of the key concepts and captures the most distinctive aspect of Nix:
that we work with a certain type data (linked files) in a certain manner (using pure functions).
Here we finally arrange all the important pieces to show how they belong
together, while referring to the respective reference documentation for details.
This change also unbreaks downstream links to `//glossary#gloss-derivation`,
which had been broken by removing the glossary entry
Nix shipping with Yet Another Programming Language is often questioned
among beginners. This change highlights distinctive aspects of the Nix
language to ease the learning curve and better orient readers around
what really matters for using Nix.
Since it's on topic, this change also polishes the wording on the motivation for string contexts.
