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/Makefile

@@ -0,0 +1,61 @@
+# Make "html" the default target, since that is what most people tend
+# to want to use.
+html:
+
+all: html
+
+subdir = doc
+repmgr_top_builddir = ..
+include $(repmgr_top_builddir)/Makefile.global
+
+XMLINCLUDE = --path .
+
+ifndef XMLLINT
+XMLLINT = $(missing) xmllint
+endif
+
+ifndef XSLTPROC
+XSLTPROC = $(missing) xsltproc
+endif
+
+override XSLTPROCFLAGS += --stringparam repmgr.version '$(REPMGR_VERSION)'
+
+ALLSGML := $(wildcard $(srcdir)/*.sgml)
+GENERATED_SGML = version.sgml
+
+##
+## HTML
+##
+
+
+html: html-stamp
+
+html-stamp: stylesheet.xsl repmgr.sgml $(ALLSGML) $(GENERATED_SGML)
+	$(XMLLINT) $(XMLINCLUDE) --noout --valid $(word 2,$^)
+	$(XSLTPROC) $(XMLINCLUDE) $(XSLTPROCFLAGS) $(XSLTPROC_HTML_FLAGS) $(wordlist 1,2,$^)
+	cp $(srcdir)/stylesheet.css html/
+	touch $@
+
+version.sgml: $(repmgr_top_builddir)/repmgr_version.h
+	{ \
+	  echo "<!ENTITY repmgrversion \"$(REPMGR_VERSION)\">"; \
+	} > $@
+
+zip: html
+	cp -r html repmgr-docs-$(REPMGR_VERSION)
+	zip -r repmgr-docs-$(REPMGR_VERSION).zip repmgr-docs-$(REPMGR_VERSION)
+	rm -rf repmgr-docs-$(REPMGR_VERSION)
+
+install: html
+	@$(MKDIR_P) $(DESTDIR)$(docdir)/$(docmoduledir)/repmgr
+	@$(INSTALL_DATA) $(wildcard html/*.html) $(wildcard html/*.css) $(DESTDIR)$(docdir)/$(docmoduledir)/repmgr
+	@echo Installed docs to $(DESTDIR)$(docdir)/$(docmoduledir)/repmgr
+
+clean:
+	rm -f html-stamp
+	rm -f HTML.index $(GENERATED_SGML)
+
+maintainer-clean:
+	rm -rf html
+
+.PHONY: html