docs(welcome): restructure documentation directories and add new guides section

docs/(welcome)/hash-table/387.mdx

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+---
+title: LC-387
+description: First Unique Character in a String
+---
+
+## Examples
+
+| Input            | Output |
+| ---------------- | ------ |
+| `"leetcode"`     | `0`    |
+| `"loveleetcode"` | `2`    |
+| `"aabb"`         | `-1`   |
+
+## Hints
+
+<Accordion type="single">
+  <AccordionItem title="Constraints">
+    Constraints:
+     • `1 <= s.length <= 10^5`
+     • `s` consists of only lowercase English letters.
+  </AccordionItem>
+  <AccordionItem title="Problem Statement">
+    Given a string `s`, find the first non-repeating character in it and return its index.
+    If it does not exist, return `-1`.
+  </AccordionItem>
+</Accordion>
+
+## Solution
+
+```js
+/**
+ * @param {string} s
+ * @return {number}
+ */
+
+var firstUniqChar = function (s) {
+  // Default for Hammer
+  const freq = {};
+  for (let c of s) freq[c] = (freq[c] || 0) + 1;
+
+  // Problem Specific
+  // Loop the ORIGINAL string — we want an index, so no sorting
+  for (let i = 0; i < s.length; i++) {
+    if (freq[s[i]] === 1) {
+      return i;
+    }
+  }
+  return -1;
+};
+```

docs/(welcome)/hash-table/451.mdx

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+---
+title: LC-451
+description: Sort Characters By Frequency
+---
+
+## Examples
+
+| Input      | Output   |
+| ---------- | -------- |
+| `"tree"`   | `"eert"` |
+| `"cccaaa"` | `"aaaccc"` |
+| `"Aabb"`   | `"bbAa"` |
+
+## Hints
+
+<Accordion type="single">
+  <AccordionItem  title="Constraints">
+    Constraints:
+     • `1 <= s.length <= 5 * 10^5`
+     • `s` consists of uppercase and lowercase English letters and digits.
+  </AccordionItem >
+  <AccordionItem  title="Problem Statement">
+    Given a string `s`, sort it in decreasing order based on the frequency of the characters.
+    The frequency of a character is the number of times it appears in the string.
+    Return the sorted string. If there are multiple answers, return any of them.
+  </AccordionItem >
+</Accordion>
+
+## Solution
+
+```js
+/**
+ * @param {string} s
+ * @return {string}
+ */
+
+var frequencySort = function (s) {
+  // Default for Hammer
+  const freq = {};
+  for (let c of s) freq[c] = (freq[c] || 0) + 1;
+
+  // Problem Specific
+  // Sort characters by descending frequency; same chars stay grouped
+  return s
+    .split("")
+    .sort((a, b) => freq[b] - freq[a] || a.localeCompare(b))
+    .join("");
+};
+```