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>

Samples/HummingbirdDocC/App/Protocols/AppArguments.swift

@@ -0,0 +1,40 @@
+// ===----------------------------------------------------------------------===
+//
+// 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
+//
+// ===----------------------------------------------------------------------===
+
+import protocol ArgumentParser.ExpressibleByArgument
+
+import struct Logging.Logger
+
+/// A protocol that defines the input arguments the sample executable requires to run.
+protocol AppArguments {
+    
+    // MARK: Properties
+    
+    /// A label given to the sample app to identify it within a communications channel.
+    var hostname: String { get }
+    
+    /// A port number assigned to the sample app from where the app either sends or receives data.
+    var port: Int { get }
+    
+    /// A log level to configure in a type that interacts with the logging system.
+    var logLevel: Logger.Level { get }
+    
+}
+
+// MARK: - Conformances
+
+/// Extends the `Logger.Level` type so it can be used as an argument.
+#if hasFeature(RetroactiveAttribute)
+extension Logger.Level: @retroactive ExpressibleByArgument {}
+#else
+extension Logger.Level: ExpressibleByArgument {}
+#endif