docs: update Lowest Common Ancestor of BST difficulty to Medium (#611)

LeetCode 235 was reclassified from Easy to Medium.

Closes #298
Closes #647

apps/website/contents/_components/QuestionGroups.json

@@ -105,7 +105,7 @@
       "url": "https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/",
       "duration": 20,
       "epi": 1,
-      "difficulty": "Easy",
+      "difficulty": "Medium",
       "id": 235,
       "topic": "binary-search-tree",
       "routines": []

apps/website/contents/best-practice-questions.md

@@ -117,7 +117,7 @@ Week 4 builds up on knowledge from previous weeks but questions are of increased
 | Question | Difficulty | LeetCode |
 | :-- | --- | --- |
 | Subtree of Another Tree | Easy | [Link](https://leetcode.com/problems/subtree-of-another-tree/) |
-| Lowest Common Ancestor of BST | Easy | [Link](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/) |
+| Lowest Common Ancestor of BST | Medium | [Link](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/) |
 | Implement Trie (Prefix Tree) | Medium | [Link](https://leetcode.com/problems/implement-trie-prefix-tree/) |
 | Add and Search Word | Medium | [Link](https://leetcode.com/problems/design-add-and-search-words-data-structure/) |
 | Kth Smallest Element in a BST | Medium | [Link](https://leetcode.com/problems/kth-smallest-element-in-a-bst/) |