docs(docs): add Approach and Explanation sections with mermaid diagrams to problem pages

apps/docs/content/(array)/1426-counting-elements.mdx

@@ -23,6 +23,19 @@ sidebar:
 - `1 <= arr.length <= 1000`
 - `0 <= arr[i] <= 1000`
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["countElements(arr)"]) --> I["arrSet = Set(arr), count = 0"]
+  I --> L{"more n in arr?"}
+  L -- no --> E(["return count"])
+  L -- yes --> H{"arrSet has n + 1?"}
+  H -- yes --> C["count++"]
+  C --> L
+  H -- no --> L
+```
+
 ## Solution
 
 ```js