docs: Work around bug in the docbook xsl stylesheets.

docbook-xsl's index generation stylesheet (autoidx.xsl) has a small bug: It
doesn't include xlink in exclude-result-prefixes. Normally just leads to a a
single xmlns:xlink in the <div> containing the index, but because our
customization emits that, xmlns:xlink intead gets added to every element
output by autoidx.xsl below the <div>, totalling around 100kB.

Adding the spurious xmlns:xlink to the <div> ourselves isn't great, but avoids
the duplication.

Reviewed-By: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/20220213201618.qz6p6noon3wagr3f%40alap3.anarazel.de

doc/src/sgml/stylesheet-html-common.xsl

@@ -127,8 +127,11 @@ set       toc,title
                                                  &uppercase;),
                                              substring(&primary;, 1, 1)))]"/>
 
-  <div class="index">
-    <!-- pgsql-docs: begin added stuff -->
+  <!-- pgsql-docs: added xmlns:xlink, autoidx.xsl doesn't include xlink in
+       exclude-result-prefixes. Without our customization that just leads to a
+       single xmlns:xlink in this div, but because we emit it it otherwise
+       gets pushed down to the elements output by autoidx.xsl -->
+  <div class="index" xmlns:xlink="http://www.w3.org/1999/xlink">
     <p class="indexdiv-quicklinks">
       <a href="#indexdiv-Symbols">
         <xsl:call-template name="gentext">