man/sd-makefs: link to btrfs.wiki.kernel.org for btrfs-man5, since the man-pages link is dead

This also adds a <citerefentry project="url"> type,
since the other btrfs manpages use man-pages/die-net and are alive,
and btrfs.w.k.o won't be used anywhere else

man/custom-html.xsl

@@ -19,7 +19,8 @@
 <xsl:template match="citerefentry[not(@project)]">
   <a>
     <xsl:attribute name="href">
-      <xsl:value-of select="refentrytitle"/><xsl:text>.html#</xsl:text>
+      <xsl:value-of select="refentrytitle"/>
+      <xsl:text>.html#</xsl:text>
       <xsl:value-of select="refentrytitle/@target"/>
     </xsl:attribute>
     <xsl:call-template name="inline.charseq"/>
@@ -133,6 +134,15 @@
   </a>
 </xsl:template>
 
+<xsl:template match="citerefentry[@project='url']">
+  <a>
+    <xsl:attribute name="href">
+      <xsl:value-of select="refentrytitle/@url"/>
+    </xsl:attribute>
+    <xsl:call-template name="inline.charseq"/>
+  </a>
+</xsl:template>
+
 <!--
   - helper template to do conflict resolution between various headings with the same inferred ID attribute/tag from the headerlink template
   - this conflict resolution is necessary to prevent malformed HTML output (multiple ID attributes with the same value)

man/systemd-makefs@.service.xml

@@ -62,8 +62,7 @@
     number specific to each file system, so only certain types are supported.
     Currently:
     <citerefentry project='man-pages'><refentrytitle>ext4</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
-    btrfs (see
-    <citerefentry project='man-pages'><refentrytitle>btrfs-man5</refentrytitle><manvolnum>5</manvolnum></citerefentry>),
+    <citerefentry project='url'><refentrytitle url='https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs(5)'>btrfs</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
     <citerefentry project='man-pages'><refentrytitle>xfs</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
     <!-- yes, that's what the man page is called. -->
     and dm-crypt partitions (see