nixos/doc: stub out epub manual

epub manuals are holding back the transition away from docbook, and
cursory research does not suggest that they are used very much. it's
still very early in the 23.11 release cycle, so if we're going to find
out just how many people do use the epub manuals it should be now.

this need not be the end of epub manuals. nixos-render-docs could be
extended to also export epubs, but that has not been done yet since it's
going to be some effort with unknown real-world usefulness.

nixos/doc/manual/default.nix

@@ -267,19 +267,41 @@ in rec {
 
   manualEpub = runCommand "nixos-manual-epub"
     { nativeBuildInputs = [ buildPackages.libxml2.bin buildPackages.libxslt.bin buildPackages.zip ];
+      doc = ''
+        <book xmlns="http://docbook.org/ns/docbook"
+              xmlns:xlink="http://www.w3.org/1999/xlink"
+              version="5.0"
+              xml:id="book-nixos-manual">
+          <info>
+            <title>NixOS Manual</title>
+            <subtitle>Version ${lib.version}</subtitle>
+          </info>
+          <chapter>
+            <title>Temporarily unavailable</title>
+            <para>
+              The NixOS manual is currently not available in EPUB format,
+              please use the <link xlink:href="https://nixos.org/nixos/manual">HTML manual</link>
+              instead.
+            </para>
+            <para>
+              If you've used the EPUB manual in the past and it has been useful to you, please
+              <link xlink:href="https://github.com/NixOS/nixpkgs/issues/237234">let us know</link>.
+            </para>
+          </chapter>
+        </book>
+      '';
+      passAsFile = [ "doc" ];
     }
     ''
       # Generate the epub manual.
       dst=$out/share/doc/nixos
 
       xsltproc \
-        ${manualXsltprocOptions} \
+        --param chapter.autolabel 0 \
         --nonet --xinclude --output $dst/epub/ \
         ${docbook_xsl_ns}/xml/xsl/docbook/epub/docbook.xsl \
-        ${manual-combined}/manual-combined.xml
+        $docPath
 
-      mkdir -p $dst/epub/OEBPS/images/callouts
-      cp -r ${docbook_xsl_ns}/xml/xsl/docbook/images/callouts/*.svg $dst/epub/OEBPS/images/callouts # */
       echo "application/epub+zip" > mimetype
       manual="$dst/nixos-manual.epub"
       zip -0Xq "$manual" mimetype