release notes: Mention automated gnomeExtensions

https://github.com/NixOS/nixpkgs/pull/118232

nixos/doc/manual/release-notes/rl-2105.xml

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      This prevents NVRAM from filling up, which ensures the latest diagnostic data is always stored and alleviates problems with writing new boot configurations.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     Nixpkgs now contains <link xlink:href="https://github.com/NixOS/nixpkgs/pull/118232">automatically packaged GNOME Shell extensions</link> from the <link xlink:href="https://extensions.gnome.org/">GNOME Extensions</link> portal. You can find them, filed by their UUID, under <literal>gnome38Extensions</literal> attribute for GNOME 3.38 and under <literal>gnome40Extensions</literal> for GNOME 40. Finally, the <literal>gnomeExtensions</literal> attribute contains extensions for the latest GNOME Shell version in Nixpkgs, listed under a more human-friendly name. The unqualified attribute scope also contains manually packaged extensions. Note that the automatically packaged extensions are provided for convenience and are not checked or guaranteed to work.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 </section>