doc: rpc_list: docbook output directory

Added directory for docbook output, complete with standard doc
makefile (for generating html, pdf, txt a.s.o from docbook
sources).

doc/rpc_list/docbook/Makefile

@@ -0,0 +1,5 @@
+
+docs = rpc_list.xml
+docbook_dir = ../../../docbook
+
+include $(docbook_dir)/Makefile