Add database index wikipedia link for Backend Roadmap (#7005)

* Add database index wikipedia link for Backend Roadmap

* Update src/data/roadmaps/backend/content/database-indexes@y-xkHFE9YzhNIX3EiWspL.md

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Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

src/data/roadmaps/backend/content/database-indexes@y-xkHFE9YzhNIX3EiWspL.md

@@ -4,5 +4,6 @@ An index is a data structure that you build and assign on top of an existing tab
 
 Visit the following resources to learn more:
 
+- [@article@Database index - Wikipedia](https://en.wikipedia.org/wiki/Database_index)
 - [@video@Database Indexing Explained](https://www.youtube.com/watch?v=-qNSXK7s7_w)
 - [@feed@Explore top posts about Database](https://app.daily.dev/tags/database?ref=roadmapsh)