refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/manual.md

@@ -0,0 +1,56 @@
+# NixOS Manual {#book-nixos-manual}
+## Version @NIXOS_VERSION@
+
+<!--
+  this is the top-level structure file for the nixos manual.
+
+  the manual structure extends the nixpkgs commonmark further with include
+  blocks to allow better organization of input text. there are six types of
+  include blocks: preface, parts, chapters, sections, appendix, and options.
+  each type except `options` corresponds to the docbook elements of (roughly)
+  the same name, and can itself can further include blocks to denote its
+  substructure.
+
+  non-`options` include blocks are fenced code blocks that list a number of
+  files to include, in the form
+
+     ```{=include=} <type>
+     <file-name-1>
+     <file-name-2>
+     <...>
+     ```
+
+  `options` include blocks do not list file names but contain a list of key-value
+  pairs that describe the options to be included and how to convert them into
+  elements of the manual output type:
+
+      ```{=include=} options
+      id-prefix: <options id prefix>
+      list-id: <variable list element id>
+      source: <path to options.json>
+      ```
+
+-->
+
+```{=include=} preface
+preface.md
+```
+
+```{=include=} parts
+installation/installation.md
+configuration/configuration.md
+administration/running.md
+development/development.md
+```
+
+```{=include=} chapters
+contributing-to-this-manual.chapter.md
+```
+
+```{=include=} appendix html:into-file=//options.html
+nixos-options.md
+```
+
+```{=include=} appendix html:into-file=//release-notes.html
+release-notes/release-notes.md
+```