Added (first version of) sample Hummingbird app. (#4)

This PR contains the work done to:
* Implemented a basic `Hummingbird` application in which to integrate the `HummingbirdDocC` library.
* Added the *ArgumentParser* package dependency to the `Package.swift` file;
* Added a new *sample* target to the `Package.swift` file;
* Added library and documentation tasks to the `Makefile` file.

Reviewed-on: #4
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Sources/HummingbirdDocC/Internal/Enumerations/DocumentationFolder.swift

@@ -0,0 +1,31 @@
+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the Hummingbird DocC Middleware open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the Hummingbird DocC Middleware project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of Hummingbird DocC Middleware project authors
+//
+// ===----------------------------------------------------------------------===
+
+/// An enumeration that represents the documentation folders that could be generated by the `DocC` building process.
+enum DocumentationFolder: String, CaseIterable {
+    /// An article document, which can also be used for (source code generated) technical documentation as well.
+    case article = "documentation"
+    /// A tutorial document.
+    case tutorial = "tutorials"
+}
+
+// MARK: - Pathable
+
+extension DocumentationFolder: Pathable {
+    
+    // MARK: Computed
+    
+    var path: String {
+        .init(format: .Format.Path.root, rawValue)
+    }
+    
+}