Add a stylesheet to make the reference HTML look a bit prettier

doc/rust.css

+body {
+  padding: 1em;
+  margin: 0;
+  font-family: "Helvetica Neue", Helvetica, sans-serif;
+}
+
+body {
+  padding: 1em 6em;
+  max-width: 50em;
+}
+
+h1 { font-size: 22pt; }
+h2 { font-size: 17pt; }
+h3 { font-size: 14pt; }
+
+pre {
+  margin: 1.1em 0;
+  padding: .4em .4em .4em 1em;
+  font-size: 120%;
+}
+
+a, a:visited, a:link {
+  text-decoration: none;
+  color: #00438a;
+}

mk/docs.mk

@@ -19,8 +19,12 @@ doc/rust.html: rust.md doc/version.md doc/keywords.md
          --section-divs \
          --number-sections \
          --from=markdown --to=html \
+         --css=rust.css \
          --output=$@ \
          $<
+	@$(call E, cp: $(S)doc/rust.css)
+	-$(Q)cp -a $(S)doc/rust.css doc/rust.css 2> /dev/null
+
 
 ifdef CFG_PDFLATEX
 
@@ -50,7 +54,7 @@ ifdef CFG_NODE
 doc/tutorial/web/index.html: \
         $(wildcard $(S)doc/tutorial/*.md)
 	@$(call E, cp: $(S)doc/tutorial)
-	$(Q)cp -a $(S)doc/tutorial doc/
+	-$(Q)cp -a $(S)doc/tutorial doc/ 2> /dev/null
 	@$(call E, node: build.js)
 	$(Q)cd doc/tutorial && $(CFG_NODE) build.js