Update Makefile

Makefile

+build:
+	julia --project=. -e 'using Pkg; Pkg.instantiate(); Pkg.resolve()'
+
+doc:
+	make -C docs/
+
+test:
+	julia --project=. -e 'using Pkg; Pkg.test()'
+
+clean:
+	rm -rf docs/build/
+
+preview:
+	julia -e 'using LiveServer; serve(dir="docs/build")'
+
+
+.PHONY: build doc test clean preview