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/Extensions/String+Constants.swift

@@ -0,0 +1,30 @@
+// ===----------------------------------------------------------------------===
+//
+// 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
+//
+// ===----------------------------------------------------------------------===
+
+extension String {
+    /// An empty string.
+    static let empty = ""
+    
+    /// A namespace that defines logging values.
+    enum Logging {
+        /// A name of the middleware that triggered a logging event.
+        static let source = "DocCMiddleware"
+    }
+    
+    /// A namespace that defines relative path values.
+    enum Path {
+        /// A forwarding slash.
+        static let forwardSlash = "/"
+        /// An indication of a previous folder in a path component.
+        static let previousFolder = ".."
+    }
+}