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

apps/docs/content/(stack)/155-min-stack.mdx

@@ -23,6 +23,28 @@ You must implement a solution with O(1) time complexity for each function.
 - Methods pop, top and getMin operations will always be called on non-empty stacks.
 - At most 3 * 10^4 calls will be made to push, pop, top, and getMin.
 
+## Approach
+
+```mermaid
+flowchart TD
+  subgraph state["two parallel stacks — same depth, always"]
+    A["stack — the values"]
+    B["minStack — the minimum as of that depth"]
+  end
+  state --> P
+  subgraph P["push(value)"]
+    P1["stack.append(value)"] --> P2["minStack.append(min(value, minStack[-1]))"]
+  end
+  state --> O
+  subgraph O["pop()"]
+    O1["pop both stacks together"]
+  end
+  state --> R
+  subgraph R["top() / getMin() — O(1)"]
+    R1(["stack[-1] / minStack[-1]"])
+  end
+```
+
 ## Solution
 
 ```py
@@ -54,3 +76,7 @@ class MinStack:
 # param_3 = obj.top()
 # param_4 = obj.getMin()
 ```
+
+## Explanation
+
+<YouTube id="qkLl7nAwDPo" title="Design Min Stack - Amazon Interview Question - Leetcode 155 - Python" />