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/AssetFolder.swift

@@ -0,0 +1,43 @@
+// ===----------------------------------------------------------------------===
+//
+// 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 all possible asset folders that could be generated by the `DocC` building process.
+enum AssetFolder: String, CaseIterable {
+    /// A folder that contains all CSS style sheets.
+    case css
+    /// A folder that contains all documentation data.
+    case data
+    /// A folder that contains all other resources.
+    case downloads
+    /// A folder that contains all image resources.
+    case images
+    /// A folder that contains all image resources.
+    case img
+    /// A folder that contains all generated `HTML` code.
+    case index
+    /// A folder that contains all generated `Javascript` code.
+    case js
+    /// A folder that contains all video resources.
+    case videos
+}
+
+// MARK: - Pathable
+
+extension AssetFolder: Pathable {
+    
+    // MARK: Computed
+    
+    var path: String {
+        .init(format: .Format.Path.folder, rawValue)
+    }
+    
+}