doc: Add index letter links to XSLT HTML build

This matches what was already implemented in the DSSSL HTML build.

doc/src/sgml/stylesheet.xsl

 <?xml version='1.0'?>
+<!DOCTYPE xsl:stylesheet [
+<!ENTITY % common.entities SYSTEM "http://docbook.sourceforge.net/release/xsl/current/common/entities.ent">
+%common.entities;
+]>
 <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                 version='1.0'
                 xmlns="http://www.w3.org/TR/xhtml1/transitional"
@@ -59,4 +63,187 @@ section   toc
 set       toc,title
 </xsl:param>
 
+
+<!-- Put index "quicklinks" (A | B | C | ...) at the top of the bookindex page. -->
+
+<!-- from html/autoidx.xsl -->
+
+<xsl:template name="generate-basic-index">
+  <xsl:param name="scope" select="NOTANODE"/>
+
+  <xsl:variable name="role">
+    <xsl:if test="$index.on.role != 0">
+      <xsl:value-of select="@role"/>
+    </xsl:if>
+  </xsl:variable>
+
+  <xsl:variable name="type">
+    <xsl:if test="$index.on.type != 0">
+      <xsl:value-of select="@type"/>
+    </xsl:if>
+  </xsl:variable>
+
+  <xsl:variable name="terms"
+                select="//indexterm
+                        [count(.|key('letter',
+                          translate(substring(&primary;, 1, 1),
+                             &lowercase;,
+                             &uppercase;))
+                          [&scope;][1]) = 1
+                          and not(@class = 'endofrange')]"/>
+
+  <xsl:variable name="alphabetical"
+                select="$terms[contains(concat(&lowercase;, &uppercase;),
+                                        substring(&primary;, 1, 1))]"/>
+
+  <xsl:variable name="others" select="$terms[not(contains(concat(&lowercase;,
+                                                 &uppercase;),
+                                             substring(&primary;, 1, 1)))]"/>
+
+  <div class="index">
+    <!-- pgsql-docs: begin added stuff -->
+    <p class="indexdiv-quicklinks">
+      <a href="#indexdiv-Symbols">
+        <xsl:call-template name="gentext">
+          <xsl:with-param name="key" select="'index symbols'"/>
+        </xsl:call-template>
+      </a>
+      <xsl:apply-templates select="$alphabetical[count(.|key('letter',
+                                   translate(substring(&primary;, 1, 1),
+                                   &lowercase;,&uppercase;))[&scope;][1]) = 1]"
+                           mode="index-div-quicklinks">
+        <xsl:with-param name="position" select="position()"/>
+        <xsl:with-param name="scope" select="$scope"/>
+        <xsl:with-param name="role" select="$role"/>
+        <xsl:with-param name="type" select="$type"/>
+        <xsl:sort select="translate(&primary;, &lowercase;, &uppercase;)"/>
+      </xsl:apply-templates>
+    </p>
+    <!-- pgsql-docs: end added stuff -->
+
+    <xsl:if test="$others">
+      <xsl:choose>
+        <xsl:when test="normalize-space($type) != '' and
+                        $others[@type = $type][count(.|key('primary', &primary;)[&scope;][1]) = 1]">
+          <!-- pgsql-docs: added id attribute here for linking to it -->
+          <div class="indexdiv" id="indexdiv-Symbols">
+            <h3>
+              <xsl:call-template name="gentext">
+                <xsl:with-param name="key" select="'index symbols'"/>
+              </xsl:call-template>
+            </h3>
+            <dl>
+              <xsl:apply-templates select="$others[count(.|key('primary', &primary;)[&scope;][1]) = 1]"
+                                   mode="index-symbol-div">
+                <xsl:with-param name="position" select="position()"/>
+                <xsl:with-param name="scope" select="$scope"/>
+                <xsl:with-param name="role" select="$role"/>
+                <xsl:with-param name="type" select="$type"/>
+                <xsl:sort select="translate(&primary;, &lowercase;, &uppercase;)"/>
+              </xsl:apply-templates>
+            </dl>
+          </div>
+        </xsl:when>
+        <xsl:when test="normalize-space($type) != ''">
+          <!-- Output nothing, as there isn't a match for $other using this $type -->
+        </xsl:when>
+        <xsl:otherwise>
+          <!-- pgsql-docs: added id attribute here for linking to it -->
+          <div class="indexdiv" id="indexdiv-Symbols">
+            <h3>
+              <xsl:call-template name="gentext">
+                <xsl:with-param name="key" select="'index symbols'"/>
+              </xsl:call-template>
+            </h3>
+            <dl>
+              <xsl:apply-templates select="$others[count(.|key('primary',
+                                          &primary;)[&scope;][1]) = 1]"
+                                  mode="index-symbol-div">
+                <xsl:with-param name="position" select="position()"/>
+                <xsl:with-param name="scope" select="$scope"/>
+                <xsl:with-param name="role" select="$role"/>
+                <xsl:with-param name="type" select="$type"/>
+                <xsl:sort select="translate(&primary;, &lowercase;, &uppercase;)"/>
+              </xsl:apply-templates>
+            </dl>
+          </div>
+        </xsl:otherwise>
+      </xsl:choose>
+    </xsl:if>
+
+    <xsl:apply-templates select="$alphabetical[count(.|key('letter',
+                                 translate(substring(&primary;, 1, 1),
+                                           &lowercase;,&uppercase;))[&scope;][1]) = 1]"
+                         mode="index-div-basic">
+      <xsl:with-param name="position" select="position()"/>
+      <xsl:with-param name="scope" select="$scope"/>
+      <xsl:with-param name="role" select="$role"/>
+      <xsl:with-param name="type" select="$type"/>
+      <xsl:sort select="translate(&primary;, &lowercase;, &uppercase;)"/>
+    </xsl:apply-templates>
+  </div>
+</xsl:template>
+
+<xsl:template match="indexterm" mode="index-div-basic">
+  <xsl:param name="scope" select="."/>
+  <xsl:param name="role" select="''"/>
+  <xsl:param name="type" select="''"/>
+
+  <xsl:variable name="key"
+                select="translate(substring(&primary;, 1, 1),
+                         &lowercase;,&uppercase;)"/>
+
+  <xsl:if test="key('letter', $key)[&scope;]
+                [count(.|key('primary', &primary;)[&scope;][1]) = 1]">
+    <div class="indexdiv">
+      <!-- pgsql-docs: added id attribute here for linking to it -->
+      <xsl:attribute name="id">
+        <xsl:value-of select="concat('indexdiv-', $key)"/>
+      </xsl:attribute>
+
+      <xsl:if test="contains(concat(&lowercase;, &uppercase;), $key)">
+        <h3>
+          <xsl:value-of select="translate($key, &lowercase;, &uppercase;)"/>
+        </h3>
+      </xsl:if>
+      <dl>
+        <xsl:apply-templates select="key('letter', $key)[&scope;]
+                                     [count(.|key('primary', &primary;)
+                                     [&scope;][1])=1]"
+                             mode="index-primary">
+          <xsl:with-param name="position" select="position()"/>
+          <xsl:with-param name="scope" select="$scope"/>
+          <xsl:with-param name="role" select="$role"/>
+          <xsl:with-param name="type" select="$type"/>
+          <xsl:sort select="translate(&primary;, &lowercase;, &uppercase;)"/>
+        </xsl:apply-templates>
+      </dl>
+    </div>
+  </xsl:if>
+</xsl:template>
+
+<!-- pgsql-docs -->
+<xsl:template match="indexterm" mode="index-div-quicklinks">
+  <xsl:param name="scope" select="."/>
+  <xsl:param name="role" select="''"/>
+  <xsl:param name="type" select="''"/>
+
+  <xsl:variable name="key"
+                select="translate(substring(&primary;, 1, 1),
+                        &lowercase;,&uppercase;)"/>
+
+  <xsl:if test="key('letter', $key)[&scope;]
+                [count(.|key('primary', &primary;)[&scope;][1]) = 1]">
+    <xsl:if test="contains(concat(&lowercase;, &uppercase;), $key)">
+      |
+      <a>
+        <xsl:attribute name="href">
+          <xsl:value-of select="concat('#indexdiv-', $key)"/>
+        </xsl:attribute>
+        <xsl:value-of select="translate($key, &lowercase;, &uppercase;)"/>
+      </a>
+    </xsl:if>
+  </xsl:if>
+</xsl:template>
+
 </xsl:stylesheet>