Open API specification improvements and License update (#17)

This PR contains the work done to:
* improve the overall `OpenAPI` specification documentation describing the `Amiibo API` service;
* update the `AmiiboFilter` type to include the `head` and `tail` properties;
* update the error handling of the errors coming up from the service;
* update its license to Apache v2.0;
* regenerate the Github Pages documentation.

Reviewed-on: #17
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

NOTICE

@@ -0,0 +1,20 @@
+                            Amiibo Service Project
+                            ======================
+
+Please visit the Amiibo Service web site for more information:
+
+  * https://github.com/rock-n-code/amiibo-service
+
+Copyright 2025 Röck+Cöde VoF
+
+The Amiibo Service Project licenses this file to you under the Apache License,
+version 2.0 (the "License"); you may not use this file except in compliance
+with the License. You may obtain a copy of the License at:
+
+  https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+License for the specific language governing permissions and limitations
+under the License.