Enable table of contents in documentation files

Oddly enough, this places the rendered HTML under the GPL, as the
tiny JavaScript snippet dumped by AsciiDoc is under the GPL, and
is included literal in the resulting HTML page.

Signed-off-by: Shawn O. Pearce <sop@google.com>

Documentation/Makefile

@@ -53,6 +53,7 @@ clean:
 $(DOC_HTML): %.html : %.txt
 	rm -f $@+ $@
 	$(ASCIIDOC) \
+		-a toc \
 		-b xhtml11 -f asciidoc.conf \
 		$(ASCIIDOC_EXTRA) -o $@+ $<
 	mv $@+ $@