docs(docs): restructure documentation directories and remove placeholder study decks

docs/solutions/23-check-if-the-sentence-is-pangram.mdx

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+---
+title: '1832. Check if the Sentence Is Pangram'
+description: A pangram is a sentence where every letter of the English alphabet appears at least once
+sidebar:
+  badge: 'Easy'
+---
+
+<Badge variant="accent">Hash Table</Badge>
+
+### Example 1:
+- Input: `sentence = "thequickbrownfoxjumpsoverthelazydog"`
+- Output: `true`
+- Explanation: `sentence` contains at least one of every letter of the English alphabet.
+
+### Example 2:
+- Input: `sentence = "leetcode"`
+- Output: `false`
+
+### Constraints:
+
+- `1 <= sentence.length <= 1000`
+- `sentence` consists of lowercase English letters.
+
+## Solution
+
+```js
+/**
+ * @param {string} sentence
+ * @return {boolean}
+ */
+var checkIfPangram = function(sentence) {
+    const freq = {};
+    for (let c of sentence) freq[c] = (freq[c] || 0) + 1;
+    
+    if (Object.keys(freq).length === 26){
+        return true;
+    }
+    return false;
+};
+```