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/Public/Configurations/DoccConfiguration.swift

@@ -0,0 +1,50 @@
+// ===----------------------------------------------------------------------===
+//
+// 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 class NIOPosix.NIOThreadPool
+
+/// A type that contains all the parameters to configure the ``DocCMiddleware`` middleware.
+public struct DocCConfiguration: Sendable {
+    
+    // MARK: Properties
+    
+    /// A path to the physical location where the `DocC` documentation containers are stored.
+    let folderRoot: String
+    
+    /// A URI path that prefixes the `DocC` documentation resources.
+    let uriRoot: String
+    
+    /// A type that define a mechanism to use in case some blocking work needs to be performed for which no non-blocking API exists.
+    let threadPool: NIOThreadPool
+    
+    // MARK: Initializers
+    
+    /// Initializes this configuration type.
+    ///
+    /// > important: It is assumed that both the `uriRoot` and the `folderRoot` parameters should not be empty, and that they should be prefixed
+    /// with the `/` forward slash character.
+    ///
+    /// - Parameters:
+    ///   - uriRoot: A URI path that prefixes the `DocC` documentation resources.
+    ///   - folderRoot: A path to the physical location where the `DocC` documentation containers are stored.
+    ///   - threadPool: A type that define a mechanism to use in case some blocking work needs to be performed for which no non-blocking API exists.
+    public init(
+        uriRoot: String,
+        folderRoot: String,
+        threadPool: NIOThreadPool = .singleton
+    ) {
+        self.folderRoot = folderRoot
+        self.uriRoot = uriRoot
+        self.threadPool = threadPool
+    }
+    
+}