Patched (temporarily) the DocC documentation (#8)

This PR contains the work done to:
* Documented the public properties, initializers, and/or functions of those types conforming to the `APIClient`, `KeyNameFilter`, and `KeyNameModel` protocols, to fix the issue that the protocol documentations cannot be inherited;
* Moved the `AmiiboService` DocC documentation catalog inside the library target;
* Amended the folder structure of the library and test targets;
* Fixed further documentation issues encountered while revising the written documentation;
* Added the `lib-test` task in the `Makefile` file;
* Improved the naming for the existing tasks in the `Makefile` file.

Reviewed-on: #8
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Sources/AmiiboService/Public/Errors/AmiiboServiceError.swift

@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===
+//
+// This source file is part of the AmiiboService open source project
+//
+// Copyright (c) 2024-2025 Röck+Cöde VoF. and the AmiiboAPI project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of AmiiboAPI project authors
+//
+//===----------------------------------------------------------------------===
+
+/// A representation of all the possible errors that the ``AmiiboService`` service could throw.
+public enum AmiiboServiceError: Error {
+    /// A bad request has been given to the client.
+    case badRequest
+    /// A response cannot be decoded.
+    case decoding
+    /// An online service is not currently available.
+    case notAvailable
+    /// A response cannot be found.
+    case notFound
+    /// An undocumented/unsupported status code error.
+    case undocumented(_ statusCode: Int)
+    /// An unknown error.
+    case unknown
+}
+
+// MARK: - Equatable
+
+extension AmiiboServiceError: Equatable {}