Nixpkgs manual: Add a Nix expression to build

doc/default.nix

@@ -0,0 +1,42 @@
+with import ./.. { };
+with lib;
+
+stdenv.mkDerivation {
+  name = "nixpkgs-manual";
+
+  sources = sourceFilesBySuffices ./. [".xml"];
+
+  buildInputs = [ libxml2 libxslt ];
+
+  xsltFlags = ''
+    --param section.autolabel 1
+    --param section.label.includes.component.label 1
+    --param html.stylesheet 'style.css'
+    --param xref.with.number.and.title 1
+    --param toc.section.depth 3
+    --param admon.style '''
+    --param callout.graphics.extension '.gif'
+  '';
+
+  buildCommand = ''
+    ln -s $sources/*.xml . # */
+
+    echo ${nixpkgsVersion} > .version
+
+    xmllint --noout --nonet --xinclude --noxincludenode \
+      --relaxng ${docbook5}/xml/rng/docbook/docbook.rng \
+      manual.xml
+
+    dst=$out/share/doc/nixpkgs
+    mkdir -p $dst
+    xsltproc $xsltFlags --nonet --xinclude \
+      --output $dst/manual.html \
+      ${docbook5_xsl}/xml/xsl/docbook/xhtml/docbook.xsl \
+      ./manual.xml
+
+    cp ${./style.css} $dst/style.css
+
+    mkdir -p $out/nix-support
+    echo "doc manual $dst manual.html" >> $out/nix-support/hydra-build-products
+  '';
+}