Implemented (a first version of) the DocCMiddleware middleware in the library target.

Library/Sources/Internal/Enumerations/IndexPrefix.swift

@@ -0,0 +1,16 @@
+enum IndexPrefix: String, CaseIterable {
+    case documentation
+    case tutorials
+}
+
+// MARK: - Pathable
+
+extension IndexPrefix: Pathable {
+    
+    // MARK: Computed
+    
+    var path: String {
+        .init(format: .Format.pathRoot, rawValue)
+    }
+    
+}