[Add] add week-2 and week-3

Author: junnei

Diff for: docs/leet-code/week-2/_category_.json

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+{
+  "position": 2,
+  "label": "Week 2",
+  "collapsible": true,
+  "collapsed": true
+}

Diff for: docs/leet-code/week-2/index.md

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+---
+sidebar_position: 200
+---
+
+# Week 2
+
+Week 2. Binary Search / Parametric Search / Sliding Window

Diff for: docs/leet-code/week-3/1.md

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+---
+sidebar_position: 1
+---
+
+# 0739-daily-temperatures
+#
+[**View Problem**](https://leetcode.com/problems/daily-temperatures)
+
+```python 0739-daily-temperatures
+print("Hello, World!")
+```

Diff for: docs/leet-code/week-3/2.md

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+---
+sidebar_position: 2
+---
+
+# 0036-valid-sudoku
+#
+[**View Problem**](https://leetcode.com/problems/valid-sudoku)
+
+```python 0036-valid-sudoku
+print("Hello, World!")
+```

Diff for: docs/leet-code/week-3/3.md

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+---
+sidebar_position: 3
+---
+
+# 0049-group-anagrams
+#
+[**View Problem**](https://leetcode.com/problems/group-anagrams)
+
+```python 0049-group-anagrams
+print("Hello, World!")
+```

Diff for: docs/leet-code/week-3/4.md

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+---
+sidebar_position: 4
+---
+
+# 0230-kth-smallest-element-in-a-bst
+#
+[**View Problem**](https://leetcode.com/problems/kth-smallest-element-in-a-bst)
+
+```python 0230-kth-smallest-element-in-a-bst
+print("Hello, World!")
+```

Diff for: docs/leet-code/week-3/_category_.json

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+{
+  "position": 3,
+  "label": "Week 3",
+  "collapsible": true,
+  "collapsed": true
+}

Diff for: docs/leet-code/week-3/index.md

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+---
+sidebar_position: 300
+---
+
+# Week 3
+
+Week 3. Stacks / Hashmaps / Trees