logo: add makefile to create PNG and PDF versions from master SVG

docs/_static/Makefile

@@ -0,0 +1,11 @@
+
+all: logo.pdf logo.png
+
+logo.pdf: logo.svg
+	inkscape logo.svg --export-pdf=logo.pdf
+
+logo.png: logo.svg
+	inkscape logo.svg --export-png=logo.png --export-dpi=72,72
+
+clean:
+	rm -f logo.pdf logo.png