manual: limit text width

Currently the manual scales to the view port of the browser.
This leads to an unreadable layout and I found myself
reading the xml source instead.
The optimal width would be around 50 characters per line.
Since we have code listings also in the manual I relaxed
this limit a bit towards 70 characters per line.

doc/configuration.xml

···
      code:
 <programlisting>
 {
-  allowUnfreePredicate = (pkg: builtins.elem (builtins.parseDrvName pkg.name).name [ "flashplayer" "vscode" ]);
+  allowUnfreePredicate = (pkg: builtins.elem
+    (builtins.parseDrvName pkg.name).name [
+      "flashplayer"
+      "vscode"
+    ]);
 }
 </programlisting>
     </para>
···
   packageOverrides = pkgs: with pkgs; {
     myPackages = pkgs.buildEnv {
       name = "my-packages";
-      paths = [ aspell bc coreutils gdb ffmpeg nixUnstable emscripten jq nox silver-searcher ];
+      paths = [
+        aspell
+        bc
+        coreutils
+        gdb
+        ffmpeg
+        nixUnstable
+        emscripten
+        jq
+        nox
+        silver-searcher
+      ];
     };
   };
 }
···
   packageOverrides = pkgs: with pkgs; {
     myPackages = pkgs.buildEnv {
       name = "my-packages";
-      paths = [ aspell bc coreutils gdb ffmpeg nixUnstable emscripten jq nox silver-searcher ];
+      paths = [
+        aspell
+        bc
+        coreutils
+        gdb
+        ffmpeg
+        nixUnstable
+        emscripten
+        jq
+        nox
+        silver-searcher
+      ];
       pathsToLink = [ "/share" "/bin" ];
     };
   };
···
   packageOverrides = pkgs: with pkgs; {
     myPackages = pkgs.buildEnv {
       name = "my-packages";
-      paths = [ aspell bc coreutils ffmpeg nixUnstable emscripten jq nox silver-searcher ];
+      paths = [
+        aspell
+        bc
+        coreutils
+        ffmpeg
+        nixUnstable
+        emscripten
+        jq
+        nox
+        silver-searcher
+      ];
       pathsToLink = [ "/share/man" "/share/doc" "/bin" ];
       extraOutputsToInstall = [ "man" "doc" ];
     };

doc/style.css

···
 body
 {
     font-family: "Nimbus Sans L", sans-serif;
+    font-size: 1em;
     background: white;
     margin: 2em 1em 2em 1em;
 }
···
 h2 /* chapters, appendices, subtitle */
 {
     font-size: 180%;
+}
+
+div.book
+{
+    text-align: center;
+}
+
+div.book > div
+{
+    /*
+     * based on https://medium.com/@zkareemz/golden-ratio-62b3b6d4282a
+     * we do 70 characters per line to fit code listings better
+     * 70 * (font-size / 1.618)
+     * expression for emacs:
+     * (* 70 (/ 1 1.618))
+     */
+    max-width: 43.2em;
+    text-align: left;
+    margin: auto;
 }
 
 /* Extra space between chapters, appendices. */
···
 {
     border: 1px solid #b0b0b0;
     padding: 3px 3px;
-    margin-left: 1.5em;
-    margin-right: 1.5em;
+    margin-left: 0.5em;
+    margin-right: 0.5em;
 
     background: #f4f4f8;
     font-family: monospace;