restructure graphical outline

incorporates insights from preliminary usability studies, observing
newcomers and people with first experience using Nix:

- software developers care about declarative environments
  - want to write a file to pass around
  - want to wrap projects in their language of choice with Nix
    - this is usally already almost impossibly hard to pull off without
      prior knowledge of Nix expression language or specifics of how the
      language ecosystem is adapted into `nixpkgs`
  - Nix expression language appears scary, but is required to do achieve
    that
- people do not actually read, but only skim

the approach here is to gently introduce Nix concepts, and talk about language
specifics much later, when people hopefully feel confident enough to get their
hands dirty with the details, breakages, and confusion.

README.md

@@ -88,7 +88,9 @@ flowchart
   end
 	first-steps --> book --> manuals --> source
 	subgraph book[The Nix Book]
-		build --> imperative-pkgs --> declarative-pkgs --> nixpkgs-learn --> dev & declarative-config
+		direction TB
+		build --> imperative-pkgs & declarative-pkgs
+		declarative-pkgs --> declarative-config & dev & nixpkgs-learn
 		subgraph build[run software]
 			direction LR
 			find-pkg[find packages]
@@ -100,14 +102,15 @@ flowchart
 			install-profile[install]
 			update
 			rollback
-			pinning
-			gc[garbage collection]
+			pin-packages[pin packages]
 		end
 		subgraph declarative-pkgs[declarative package management]
 			direction LR
 			decl-env[reproducible environment]
 			compose[compose packages]
 			adapt[adapt packages]
+			pin-deps[pin dependencies]
+			gc[garbage collection]
 		end
 		subgraph nixpkgs-learn[Nix package collection]
 			direction LR
@@ -121,8 +124,9 @@ flowchart
 			os[operating system distribution]
 			services[service management]
 		end
-		subgraph dev[software development with Nix]
+		subgraph dev[software development]
 			direction LR
+			lang-ecosystems[language ecosystems]
 			ci[continuous integration]
 			distributed[distributed builds]
 			caching