Implemented the Authentication middleware (#3)

This PR contains the work done to implement the `AuthMiddleware` middleware, to authenticate the requests sent to the backend service, based on [their specifications](https://www.discogs.com/developers/#page:authentication).

In addition, some documentation has been added/updated and some boilerplate source code has been removed from the project.

Reviewed-on: #3
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Sources/DiscogsService/Catalogs/DiscogsService.docc/Library.md

@@ -8,6 +8,26 @@
 
 ## Topics
 
-### <!--@START_MENU_TOKEN@-->Group<!--@END_MENU_TOKEN@-->
+### Clients
 
-- <!--@START_MENU_TOKEN@-->``Symbol``<!--@END_MENU_TOKEN@-->
+- ``Client``
+
+### Servers
+
+- ``Servers/Server1``
+
+### Authentication
+
+- ``AuthMiddleware``
+- ``AuthMethod``
+- ``AuthTransport``
+
+### Types
+
+- ``Components``
+- ``Operations``
+- ``Servers``
+
+### Protocols
+
+- ``APIProtocol``