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/AssetFile.swift

@@ -0,0 +1,38 @@
+// ===----------------------------------------------------------------------===
+//
+// 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 essential static files that could be generated by the `DocC` building process.
+enum AssetFile: String, CaseIterable {
+    /// A file defining all the documentation available, which will be used to redirect to the root of the documentation's root article.
+    case documentation = "documentation.json"
+    /// A file containing the icon in `.ico` format within the documentation generated by the `DocC` building process.
+    case faviconICO = "favicon.ico"
+    /// A file containing the icon in `.svg` format within the documentation generated by the `DocC` building process.
+    case faviconSVG = "favicon.svg"
+    /// A file containing the theme settings within the documentation generated by the `DocC` building process.
+    case themeSettings = "theme-settings.json"
+}
+
+// MARK: - Pathable
+
+extension AssetFile: Pathable {
+    
+    // MARK: Computed
+    
+    var path: String {
+        switch self {
+        case .documentation: .init(format: .Format.Path.data, rawValue)
+        default: .init(format: .Format.Path.root, rawValue)
+        }
+    }
+    
+}

Sources/HummingbirdDocC/Internal/Enumerations/AssetFolder.swift

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+// ===----------------------------------------------------------------------===
+//
+// 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)
+    }
+    
+}

Sources/HummingbirdDocC/Internal/Enumerations/DocumentationFolder.swift

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+// ===----------------------------------------------------------------------===
+//
+// 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)
+    }
+    
+}