Add self-contained static site build target

This target is self-contained and produces a site.tar.gz output that
contains the entire static site. We can deploy the contents of
site.tar.gz to the gh-pages branch.

Signed-off-by: Torin Sandall <torinsandall@gmail.com>

docs/Makefile

@@ -1,3 +1,18 @@
+all:
+ifndef VERSION
+$(error VERSION is not set)
+endif
+	rm -fr public resources generated node_modules
+	echo "- $(VERSION)" > data/releases.yaml
+	mkdir -p generated/docs
+	cp -r content/docs generated/docs/v$(VERSION)
+	npm install
+	hugo \
+		--contentDir generated \
+		--ignoreCache \
+		--minify
+	tar czvf site.tar.gz -C public .
+
 clean:
 	rm -rf public