nixos/doc: add 21.11 release notes stub

nixos/doc/manual/from_md/release-notes/rl-2111.section.xml

@@ -0,0 +1,15 @@
+<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="release-21.11">
+  <title>Release 21.11 (<quote>?</quote>, 2021.11/??)</title>
+  <para>
+    In addition to numerous new and upgraded packages, this release has
+    the following highlights:
+  </para>
+  <itemizedlist spacing="compact">
+    <listitem>
+      <para>
+        Support is planned until the end of April 2022, handing over to
+        22.05.
+      </para>
+    </listitem>
+  </itemizedlist>
+</section>

nixos/doc/manual/release-notes/release-notes.xml

@@ -8,6 +8,7 @@
   This section lists the release notes for each stable version of NixOS and
   current unstable revision.
  </para>
+ <xi:include href="../from_md/release-notes/rl-2111.section.xml" />
  <xi:include href="rl-2105.xml" />
  <xi:include href="rl-2009.xml" />
  <xi:include href="rl-2003.xml" />

nixos/doc/manual/release-notes/rl-2111.section.md

@@ -0,0 +1,5 @@
+# Release 21.11 (“?”, 2021.11/??) {#release-21.11}
+
+In addition to numerous new and upgraded packages, this release has the following highlights:
+
+* Support is planned until the end of April 2022, handing over to 22.05.