Update docs

docs/Makefile

@@ -0,0 +1,7 @@
+DOXYGEN_INDEX = doxygen/html/index.html
+
+all: $(DOXYGEN_INDEX)
+
+$(DOXYGEN_INDEX):
+	doxygen Doxyfile
+

docs/shell.nix

@@ -0,0 +1,9 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell rec {
+
+  buildInputs = with pkgs; [
+    doxygen
+  ];
+
+}