nixos docs: Give sections IDs

nixos/doc/manual/configuration/linux-kernel.xml

···
   sets the kernel’s TCP keepalive time to 120 seconds. To see the available
   parameters, run <command>sysctl -a</command>.
  </para>
- <section>
+ <section xml:id="sec-linux-config-customizing">
   <title>Customize your kernel</title>
 
   <para>
    The first step before compiling the kernel is to generate an appropriate
-   <literal>.config</literal> configuration. Either you pass your own config via
-   the <literal>configfile</literal> setting of <literal>linuxManualConfig</literal>:
-  <screen><![CDATA[
+   <literal>.config</literal> configuration. Either you pass your own config
+   via the <literal>configfile</literal> setting of
+   <literal>linuxManualConfig</literal>:
+<screen><![CDATA[
   custom-kernel = super.linuxManualConfig {
     inherit (super) stdenv hostPlatform;
     inherit (linux_4_9) src;
···
   ]]></screen>
   </para>
  </section>
- <section>
+ <section xml:id="sec-linux-config-developing-modules">
   <title>Developing kernel modules</title>
 
   <para>

nixos/doc/manual/development/assertions.xml

···
   NixOS module system.
  </para>
 
- <section>
+ <section xml:id="sec-assertions-warnings">
   <title>Warnings</title>
 
   <para>
···
 </programlisting>
  </section>
 
- <section>
+ <section xml:id="sec-assertions-assertions">
   <title>Assertions</title>
 
   <para>

nixos/doc/manual/development/option-types.xml

···
   value definitions.
  </para>
 
- <section>
+ <section xml:id="sec-option-types-basic">
   <title>Basic Types</title>
 
   <para>
···
   </variablelist>
  </section>
 
- <section>
+ <section xml:id="sec-option-types-value">
   <title>Value Types</title>
 
   <para>
···
   </variablelist>
  </section>
 
- <section>
+ <section xml:id="sec-option-types-composed">
   <title>Composed Types</title>
 
   <para>
···
   </example>
  </section>
 
- <section>
+ <section xml:id="sec-option-types-extending">
   <title>Extending types</title>
 
   <para>
···
   </variablelist>
  </section>
 
- <section>
+ <section xml:id="sec-option-types-custom">
   <title>Custom Types</title>
 
   <para>
    Custom types can be created with the <literal>mkOptionType</literal>
    function. As type creation includes some more complex topics such as
    submodule handling, it is recommended to get familiar with
-   <filename 
+   <filename
   xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/types.nix">types.nix</filename>
    code before creating a new type.
   </para>

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

···
   sources and presenting it in an accessible style would be a worthy
   contribution to the project.
  </para>
- <section>
+ <section xml:id="sec-writing-docs-building-the-manual">
   <title>Building the Manual</title>
 
   <para>
···
    <filename>./result/share/doc/nixos/index.html</filename>.
   </para>
  </section>
- <section>
+ <section xml:id="sec-writing-docs-editing-docbook-xml">
   <title>Editing DocBook XML</title>
 
   <para>
···
    Issue</link> and someone will handle the conversion to XML for you.
   </para>
  </section>
- <section>
+ <section xml:id="sec-writing-docs-creating-a-topic">
   <title>Creating a Topic</title>
 
   <para>
···
    </itemizedlist>
   </para>
  </section>
- <section>
+ <section xml:id="sec-writing-docs-adding-a-topic">
   <title>Adding a Topic to the Book</title>
 
   <para>

nixos/doc/manual/installation/upgrading.xml

···
    so in that case you will not be able to go back to your original channel.
   </para>
  </warning>
- <section>
+ <section xml:id="sec-upgrading-automatic">
   <title>Automatic Upgrades</title>
 
   <para>