nixos/doc/md-to-db.sh: handle Docbook inclues in CommonMark

You can do includes like this:

```{=docbook}
<xi:include href="rl-2111.section.xml" />
```

nixos/doc/manual/from_md/development/writing-documentation.chapter.xml

@@ -1,4 +1,4 @@
-<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-writing-documentation">
+<chapter xmlns="http://docbook.org/ns/docbook"  xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xi="http://www.w3.org/2001/XInclude" xml:id="sec-writing-documentation">
   <title>Writing NixOS Documentation</title>
   <para>
     As NixOS grows, so too does the need for a catalogue and explanation

nixos/doc/manual/md-to-db.sh

@@ -24,15 +24,19 @@ mapfile -t MD_FILES < <(find . -type f -regex '.*\.md$')
 for mf in ${MD_FILES[*]}; do
   if [ "${mf: -11}" == ".section.md" ]; then
     mkdir -p $(dirname "$OUT/$mf")
+    OUTFILE="$OUT/${mf%".section.md"}.section.xml"
     pandoc "$mf" "${pandoc_flags[@]}" \
-      -o "$OUT/${mf%".section.md"}.section.xml"
+      -o "$OUTFILE"
+    grep -q -m 1 "xi:include" "$OUTFILE" && sed -i 's|xmlns:xlink="http://www.w3.org/1999/xlink"| xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xi="http://www.w3.org/2001/XInclude"|' "$OUTFILE"
   fi
 
   if [ "${mf: -11}" == ".chapter.md" ]; then
     mkdir -p $(dirname "$OUT/$mf")
+    OUTFILE="$OUT/${mf%".chapter.md"}.chapter.xml"
     pandoc "$mf" "${pandoc_flags[@]}" \
       --top-level-division=chapter \
-      -o "$OUT/${mf%".chapter.md"}.chapter.xml"
+      -o "$OUTFILE"
+    grep -q -m 1 "xi:include" "$OUTFILE" && sed -i 's|xmlns:xlink="http://www.w3.org/1999/xlink"| xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xi="http://www.w3.org/2001/XInclude"|' "$OUTFILE"
   fi
 done