docs: rename INDEX.md to index.md and update display name

- Renamed root INDEX.md to index.md (lowercase)
- Changed navigation title from "Documentation Index" to "Index"
- Removed caps lock for cleaner appearance

internal/docs/navigation.go

@@ -104,8 +104,8 @@ func (h *DocsHandler) getDocTitle(docPath string) string {
 
 	// Handle special cases
 	switch filename {
-	case "INDEX.md":
-		return "Documentation Index"
+	case "index.md":
+		return "Index"
 	case "user/user-guide.md":
 		return "User Guide"
 	case "developer/development.md":