Use absolute links for 404 page

Fixes #12

Signed-off-by: Richard Laager <rlaager@wiktel.com>

docs/Makefile

@@ -13,6 +13,14 @@ help:
 
 .PHONY: help Makefile
 
+html: Makefile
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	# The 404 page can be served at any directory level, so its links must
+	# be relative to the root.  Also, this command must be idempotent, as
+	# 404.html is not rebuilt if it already exists and 404.rst was not
+	# changed.
+	@sed -Ei 's#(src|href)="([^/])#\1="/\2#' "$(BUILDDIR)/html/404.html"
+
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile