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

apps/docs/content/(hash-table)/146-lru-cache.mdx

@@ -20,6 +20,31 @@ sidebar:
 - `0 <= value <= 10^5`
 - At most 2 * 10^5 calls will be made to get and put.
 
+## Approach
+
+```mermaid
+flowchart TD
+  subgraph state["state — dict for O(1) lookup, doubly linked list for O(1) reorder"]
+    H["head — least recent"] --- N["... nodes ..."] --- T["tail — most recent"]
+  end
+  subgraph get["get(key)"]
+    G1{"key in cache?"} -- no --> G2(["return -1"])
+    G1 -- yes --> G3["_remove_node then _add_node — move it beside the tail"]
+    G3 --> G4(["return node.value"])
+  end
+  subgraph put["put(key, value)"]
+    P1{"key in cache?"} -- yes --> P2["update value, _remove_node then _add_node"]
+    P1 -- no --> P3["new Node, cache[key] = node, _add_node"]
+    P3 --> P4{"len(cache) > capacity?"}
+    P4 -- yes --> P5["evict head.next — the least recent — and del cache[lru.key]"]
+    P4 -- no --> P6(["done"])
+    P2 --> P6
+    P5 --> P6
+  end
+  state --> get
+  state --> put
+```
+
 ## Solution
 
 ```py
@@ -84,3 +109,7 @@ class LRUCache:
 # param_1 = obj.get(key)
 # obj.put(key,value)
 ```
+
+## Explanation
+
+<YouTube id="7ABFKPK2hD4" title="LRU Cache - Twitch Interview Question - Leetcode 146" />