docs: update docs

docs/developer-manual/index.md

@@ -0,0 +1,10 @@
+---
+title: User Manual
+nav_order: 4
+---
+
+# User Manual
+{: .no_toc }
+
+CSS utility classes come in handy when you want to override default styles to create additional whitespace (margins/padding), correct unexpected shifts in font size or weight, add color, or hide (or show) something at a specific screen size.
+{: .fs-6 .fw-300 }