design -> architecture, add motivation

following ideas found in Architecture of Gazelle[1]

[1]: 56d35f8db0/Design.rst

doc/manual/src/SUMMARY.md.in

@@ -15,7 +15,7 @@
     - [Multi-User Mode](installation/multi-user.md)
   - [Environment Variables](installation/env-variables.md)
   - [Upgrading Nix](installation/upgrading.md)
-- [Design and Data Model](design/design.md)
+- [Architecture](architecture/architecture.md)
   - [Store](design/store/store.md)
     - [Store Objects](design/store/objects.md)
     - [Store Paths](design/store/paths.md)

doc/manual/src/architecture/architecture.md

@@ -1,8 +1,9 @@
-# Design and Data Model
+# Architecture
 
-This chapter is about the technical principles behind Nix.
+This chapter describes how Nix works.
+It should help users understand why Nix behaves as it does, and it should help developers understand how to modify Nix and how to write similar tools.
 
-## Architecture
+## Overview
 
 Nix consists of hierarchical [layers](https://en.m.wikipedia.org/wiki/Multitier_architecture#Layers).