man: use manpages.ubuntu.com for resolvconf(8) link

Add manpages.debian.org as a man citeref "project", so we can refer to man
pages hosted there. resolvconf(8) doesn't seem to appear on any of the ones we
currently have defined.

We are doing a trick, where our man page is installed as resolvconf(1), and
we refer the reader to resolvconf(8). This can be pretty confusing, so the
least we can do is to provide a non-broken symlink ;)

v2:
- link to manpages.d.o instead of manpages.ubuntu.com, because debian is the
  upstream here, and because with debian we can use /unstable/ as the version,
  and ubuntu doesn't seem to have any such shortcut, so the version would need
  to be periodically updated.

man/custom-html.xsl

@@ -1,9 +1,5 @@
 <?xml version='1.0'?> <!--*-nxml-*-->
-
-<!--
-  SPDX-License-Identifier: LGPL-2.1+
-
--->
+<!-- SPDX-License-Identifier: LGPL-2.1+ -->
 
 <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
 
@@ -96,6 +92,21 @@
   </a>
 </xsl:template>
 
+<xsl:template match="citerefentry[@project='debian']">
+  <a>
+    <xsl:attribute name="href">
+      <xsl:text>https://manpages.debian.org/unstable/</xsl:text>
+      <xsl:value-of select="refentrytitle"/>
+      <xsl:text>/</xsl:text>
+      <xsl:value-of select="refentrytitle"/>
+      <xsl:text>.</xsl:text>
+      <xsl:value-of select="manvolnum"/>
+      <xsl:text>.en.html</xsl:text>
+    </xsl:attribute>
+    <xsl:call-template name="inline.charseq"/>
+  </a>
+</xsl:template>
+
 <xsl:template match="citerefentry[@project='freebsd']">
   <a>
     <xsl:attribute name="href">

man/resolvectl.xml

@@ -288,11 +288,13 @@
   </refsect1>
 
   <refsect1>
-    <title>Compatibility with <citerefentry><refentrytitle>resolvconf</refentrytitle><manvolnum>8</manvolnum></citerefentry></title>
+    <title>Compatibility with
+    <citerefentry project="debian"><refentrytitle>resolvconf</refentrytitle><manvolnum>8</manvolnum></citerefentry></title>
 
     <para><command>resolvectl</command> is a multi-call binary. When invoked as <literal>resolvconf</literal>
     (generally achieved by means of a symbolic link of this name to the <command>resolvectl</command> binary) it
-    is run in a limited <citerefentry><refentrytitle>resolvconf</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+    is run in a limited
+    <citerefentry project="debian"><refentrytitle>resolvconf</refentrytitle><manvolnum>8</manvolnum></citerefentry>
     compatibility mode. It accepts mostly the same arguments and pushes all data into
     <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
     similar to how <option>dns</option> and <option>domain</option> commands operate. Note that
@@ -367,7 +369,9 @@
 
     </variablelist>
 
-    <para>See <citerefentry><refentrytitle>resolvconf</refentrytitle><manvolnum>8</manvolnum></citerefentry> for details on this command line options.</para>
+    <para>See
+    <citerefentry project="debian"><refentrytitle>resolvconf</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+    for details on those command line options.</para>
   </refsect1>
 
   <refsect1>