doc: define entity &releasedate;

This should be used wherever we need to show the latest release
date.

Don't use this in the release notes however, as it will be easy to
forget to update it when adding notes for a new release.

doc/Makefile

@@ -31,6 +31,7 @@ ALLXML := $(wildcard $(srcdir)/*.xml) $(GENERATED_XML)
 version.xml: $(repmgr_top_builddir)/repmgr_version.h
 	{ \
 	  echo "<!ENTITY repmgrversion \"$(REPMGR_VERSION)\">"; \
+	  echo "<!ENTITY releasedate \"$(REPMGR_RELEASE_DATE)\">"; \
 	} > $@
 
 ##