doc: compile twice the documentation to update the TOC

If you compile once the documentation, the table of contents is empty
You have to compile twice to fill the table of contents.

documentation/Makefile

@@ -55,6 +55,8 @@ pdf: $(EXT_FILES)
 	$(MAKE) -C images
 	$(MAKE) -C graphs
 	pdflatex $(LATEX_OPTIONS) $(FILE)
+# compile twice to update the table of contents
+	pdflatex $(LATEX_OPTIONS) $(FILE)
 
 index:
 	makeindex $(FILE)