nixos docs: give IDs to things

nixos/modules/security/acme.xml

@@ -11,7 +11,7 @@ retrieval and renewal using the ACME protocol. This is currently only
 implemented by and for Let's Encrypt. The alternative ACME client
 <literal>simp_le</literal> is used under the hood.</para>
 
-<section><title>Prerequisites</title>
+<section xml:id="module-security-acme-prerequisites"><title>Prerequisites</title>
 
 <para>You need to have a running HTTP server for verification. The server must
 have a webroot defined that can serve
@@ -41,7 +41,7 @@ http {
 
 </section>
 
-<section><title>Configuring</title>
+<section xml:id="module-security-acme-configuring"><title>Configuring</title>
 
 <para>To enable ACME certificate retrieval &amp; renewal for a certificate for
 <literal>foo.example.com</literal>, add the following in your
@@ -66,7 +66,7 @@ options for the <link linkend="opt-security.acme.certs">security.acme</link> mod
 
 </section>
 
-<section><title>Using ACME certificates in Nginx</title>
+<section xml:id="module-security-acme-nginx"><title>Using ACME certificates in Nginx</title>
 <para>NixOS supports fetching ACME certificates for you by setting
   <literal><link linkend="opt-services.nginx.virtualHosts._name_.enableACME">enableACME</link> = true;</literal> in a virtualHost config. We
 first create self-signed placeholder certificates in place of the