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/SampleApp.swift

@@ -0,0 +1,48 @@
+// ===----------------------------------------------------------------------===
+//
+// 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.AsyncParsableCommand
+
+import struct ArgumentParser.OptionGroup
+import struct Logging.Logger
+
+/// A type that implements and runs the sample executable that showcases the `Hummingbird-DocC` middleware.
+@main struct SampleApp {
+    
+    // MARK: Properties
+    
+    /// A type that contains all the necessary input parameters to run the sample executable.
+    @OptionGroup var arguments: Arguments
+
+}
+
+// MARK: - AsyncParsableCommand
+
+extension SampleApp: AsyncParsableCommand {
+    
+    // MARK: Functions
+    
+    func run() async throws {
+        let builder = AppBuilder(logger: {
+            var logger = Logger(label: "sample.hummingbird-docc.logger")
+            
+            logger.logLevel = arguments.logLevel
+            
+            return logger
+        }())
+        
+        let application = builder(arguments)
+
+        try await application.run()
+    }
+    
+}