add skill_tree to doc.json

doc/doc.json

@@ -5,6 +5,7 @@
     "en": "https://docs.opencv.org/4.x/"
   },
   "ask": "https://ask.csdn.net/channel/14/tag/772",
+  "skill_tree": "https://opencv.csdn.net",
   "versions": [
     {
       "released_at": "2021-10-11",