doc: convert documentation to DocBook XML

This brings the repmgr documentation build system in line with that
used by the main PostgreSQL project, and removed the restriction
that documentation must be built against PostgreSQL 9.6 or earlier.

Main formatting changes are:

 - convert empty-element tags (mainly <xref/>)
 - put <indexterm> sections in the correct location
 - correct usage of various entities.

doc/stylesheet-common.xsl

@@ -0,0 +1,89 @@
+<?xml version="1.0" encoding="utf-8"?>
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
+                version="1.0">
+
+<!--
+  This file contains XSLT stylesheet customizations that are common to
+  all output formats (HTML, HTML Help, XSL-FO, etc.).
+  -->
+
+<xsl:include href="stylesheet-speedup-common.xsl" />
+
+<!-- Parameters -->
+
+<!--
+<xsl:param name="draft.mode">
+  <xsl:choose>
+    <xsl:when test="contains($repmgr.version, 'devel')">yes</xsl:when>
+    <xsl:otherwise>no</xsl:otherwise>
+  </xsl:choose>
+</xsl:param>
+-->
+
+<xsl:param name="show.comments">
+  <xsl:choose>
+    <xsl:when test="contains($repmgr.version, 'devel')">1</xsl:when>
+    <xsl:otherwise>0</xsl:otherwise>
+  </xsl:choose>
+</xsl:param>
+
+<xsl:param name="callout.graphics" select="'0'"></xsl:param>
+<xsl:param name="toc.section.depth">2</xsl:param>
+<xsl:param name="linenumbering.extension" select="'0'"></xsl:param>
+<xsl:param name="section.autolabel" select="1"></xsl:param>
+<xsl:param name="section.label.includes.component.label" select="1"></xsl:param>
+<xsl:param name="refentry.generate.name" select="0"></xsl:param>
+<xsl:param name="refentry.generate.title" select="1"></xsl:param>
+<xsl:param name="refentry.xref.manvolnum" select="0"/>
+<xsl:param name="formal.procedures" select="0"></xsl:param>
+<xsl:param name="generate.consistent.ids" select="1"/>
+<xsl:param name="punct.honorific" select="''"></xsl:param>
+<xsl:param name="variablelist.term.break.after">1</xsl:param>
+<xsl:param name="variablelist.term.separator"></xsl:param>
+<xsl:param name="xref.with.number.and.title" select="0"></xsl:param>
+
+
+<!-- Change display of some elements -->
+
+<xsl:template match="productname">
+  <xsl:call-template name="inline.charseq"/>
+</xsl:template>
+
+<xsl:template match="structfield">
+  <xsl:call-template name="inline.monoseq"/>
+</xsl:template>
+
+<xsl:template match="structname">
+  <xsl:call-template name="inline.monoseq"/>
+</xsl:template>
+
+<xsl:template match="symbol">
+  <xsl:call-template name="inline.monoseq"/>
+</xsl:template>
+
+<xsl:template match="systemitem">
+  <xsl:call-template name="inline.charseq"/>
+</xsl:template>
+
+<xsl:template match="token">
+  <xsl:call-template name="inline.monoseq"/>
+</xsl:template>
+
+<xsl:template match="type">
+  <xsl:call-template name="inline.monoseq"/>
+</xsl:template>
+
+<xsl:template match="programlisting/emphasis">
+  <xsl:call-template name="inline.boldseq"/>
+</xsl:template>
+
+
+<!-- Special support for Tcl synopses -->
+
+<xsl:template match="optional[@role='tcl']">
+  <xsl:text>?</xsl:text>
+  <xsl:call-template name="inline.charseq"/>
+  <xsl:text>?</xsl:text>
+</xsl:template>
+
+</xsl:stylesheet>