zk: Fix note dir location and add cd alias

The note dir is in fact our `$WIKIROOT` so we set it to that by default.
Also took the chance and added a small 'cd' command into the note dir,
using `ncd`

writing/zk/config/sh/alias.d/zk.sh

@@ -15,3 +15,5 @@ n() {
 if command -v nvim >/dev/null 2>&1; then
     alias ni='nvim +"lua require \"zk.commands\".get(\"ZkCd\")()" +"edit $WIKIROOT/index.md"'
 fi
+
+alias ncd='pushd $WIKIROOT'