nixos/doc: convert "Contributing to this manual" to CommonMark

Also updates it to mention running md-to-db.sh.

nixos/doc/manual/contributing-to-this-manual.chapter.md

···
+# Contributing to this manual {#chap-contributing}
+
+The DocBook and CommonMark sources of NixOS' manual are in the [nixos/doc/manual](https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual) subdirectory of the [Nixpkgs](https://github.com/NixOS/nixpkgs) repository.
+
+You can quickly check your edits with the following:
+
+```ShellSession
+$ cd /path/to/nixpkgs
+$ ./nixos/doc/manual/md-to-db.sh
+$ nix-build nixos/release.nix -A manual.x86_64-linux
+```
+
+If the build succeeds, the manual will be in `./result/share/doc/nixos/index.html`.

nixos/doc/manual/contributing-to-this-manual.xml

···
-<chapter xmlns="http://docbook.org/ns/docbook"
-         xmlns:xlink="http://www.w3.org/1999/xlink"
-         xml:id="chap-contributing">
- <title>Contributing to this manual</title>
- <para>
-  The DocBook sources of NixOS' manual are in the <filename
-xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual">
-nixos/doc/manual</filename> subdirectory of the <link
-xlink:href="https://github.com/NixOS/nixpkgs">Nixpkgs</link> repository.
- </para>
- <para>
-  You can quickly check your edits with the following:
- </para>
-<screen>
-<prompt>$ </prompt>cd /path/to/nixpkgs
-<prompt>$ </prompt>nix-build nixos/release.nix -A manual.x86_64-linux
-</screen>
- <para>
-  If the build succeeds, the manual will be in
-  <filename>./result/share/doc/nixos/index.html</filename>.
- </para>
-</chapter>

nixos/doc/manual/from_md/contributing-to-this-manual.chapter.xml

···
+<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="chap-contributing">
+  <title>Contributing to this manual</title>
+  <para>
+    The DocBook and CommonMark sources of NixOS’ manual are in the
+    <link xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual">nixos/doc/manual</link>
+    subdirectory of the
+    <link xlink:href="https://github.com/NixOS/nixpkgs">Nixpkgs</link>
+    repository.
+  </para>
+  <para>
+    You can quickly check your edits with the following:
+  </para>
+  <programlisting>
+$ cd /path/to/nixpkgs
+$ ./nixos/doc/manual/md-to-db.sh
+$ nix-build nixos/release.nix -A manual.x86_64-linux
+</programlisting>
+  <para>
+    If the build succeeds, the manual will be in
+    <literal>./result/share/doc/nixos/index.html</literal>.
+  </para>
+</chapter>

nixos/doc/manual/manual.xml

···
   <xi:include href="./generated/options-db.xml"
                 xpointer="configuration-variable-list" />
  </appendix>
- <xi:include href="contributing-to-this-manual.xml" />
+ <xi:include href="./from_md/contributing-to-this-manual.chapter.xml" />
  <xi:include href="release-notes/release-notes.xml" />
 </book>

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

···
 mapfile -t MD_FILES < <(find . -type f -regex '.*\.md$')
 
 for mf in ${MD_FILES[*]}; do
-  mkdir -p $(dirname "$OUT/$mf")
   if [ "${mf: -11}" == ".section.md" ]; then
+    mkdir -p $(dirname "$OUT/$mf")
     pandoc "$mf" -t docbook \
       --extract-media=media \
       -f markdown+smart \
···
   fi
 
   if [ "${mf: -11}" == ".chapter.md" ]; then
+    mkdir -p $(dirname "$OUT/$mf")
     pandoc "$mf" -t docbook \
       --top-level-division=chapter \
       --extract-media=media \