docs(docs): add ProblemBadges component, layout slot, and enrich pages with topics/video frontmatter, plus guide placeholders and topic‑landing map.

apps/docs/content/(01-linear-structures)/stack/20-valid-parentheses.mdx

@@ -2,13 +2,16 @@
 title: '20. Valid Parentheses'
 description: Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid
 icon: dot
+topics:
+  - { name: "String", slug: "string" }
+  - { name: "Stack", slug: "stack" }
+  - { name: "Bracket Sequences", slug: "bracket-sequences" }
+video: "WTzjTskDFMg"
 sidebar:
   label: 'Valid Parentheses'
   badge: 'Easy'
 ---
 
-<Badge variant="accent">String</Badge>
-
 ### Example 1:
 - Input: `s = "()"`
 - Output: `true`
@@ -71,7 +74,3 @@ class Solution:
 
         return True if not stack else False
 ```
-
-## Explanation
-
-<YouTube id="WTzjTskDFMg" title="Valid Parentheses - Stack - Leetcode 20 - Python" />