Makefile: also clean contrib

... and also adjust README and INITIAL-COMMANDS.

Makefile

@@ -43,4 +43,4 @@ rsc: $(GEN_RSC)
 
 clean:
 	rm -f $(HTML) checksums.json
-	make -C contrib clean
+	make -C contrib/ clean

general/clipboard.js

@@ -1,8 +1,8 @@
 /* copy code to clipboard */
-function CopyToClipboard(pre) {
-  pre.style.filter = 'invert(1)';
-  navigator.clipboard.writeText(pre.firstElementChild.innerHTML);
+function CopyToClipboard(element) {
+  element.style.filter = 'invert(1)';
+  navigator.clipboard.writeText(element.firstElementChild.innerHTML);
   setTimeout(function() {
-    pre.style.filter = 'invert(0)';
+    element.style.filter = 'invert(0)';
   }, 100);
 }