DocC documentation support (#4)

This PR contains the work done to:
* Documented all the `private`, `internal`, and `public` interfaces on the existing codebase;
* Set the DocC documentation catalog in the project;
* Written the main `Library` article for the DocC documentation catalog;
* Added the documentation tasks in 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/Public/Models/GameSeries.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===
 
+/// A model that represents a game series.
 public struct GameSeries: KeyNameModel {
     
     // MARK: Properties
@@ -17,7 +18,7 @@ public struct GameSeries: KeyNameModel {
     public let key: String
     public let name: String
     
-    // MARK: Initialisers
+    // MARK: Initializers
     
     init(_ payload: Components.Schemas.Tuple) {
         self.key = payload.key