Implemented the Services and Database GET endpoints (#2)

This PR contains the work done to include the define the information sections of the `OpenAPI` specification document and all the `GET` endpoints of the _Service_ and _Database_ categories.

The following endpoints have been defined in the document, and also tested manually with `Swift`:
* GET `/`
* GET `/artists/{artist_id}`
* GET `/artists/{artist_id}/releases`
* GET `/labels/{label_id}`
* GET `/labels/{label_id}/releases`
* GET `/masters/{master_id}`
* GET `/masters/{master_id}/versions`
* GET `/releases/{release_id}`
* GET `/releases/{release_id}/rating`
* GET `/releases/{release_id}/rating/{username}`
* GET `/releases/{release_id}/stats`
* GET `/database/search`

In addition, a first version of the _Github Pages_ documentation has been generated from the `DocC` documentation catalog, and the generated Swift code from the `openapi.yaml` file.

Reviewed-on: #2
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Package.swift

@@ -32,12 +32,22 @@ let package = Package(
         ),
     ],
     dependencies: [
+        .package(url: "https://github.com/apple/swift-openapi-generator.git", from: "1.3.0"),
+        .package(url: "https://github.com/apple/swift-openapi-runtime", from: "1.5.0"),
+        .package(url: "https://github.com/apple/swift-openapi-urlsession", from: "1.0.2"),
         .package(url: "https://github.com/swiftlang/swift-docc-plugin", from: "1.1.0")
     ],
     targets: [
         .target(
             name: DiscogsService.target,
-            path: "Sources/\(DiscogsService.target)"
+            dependencies: [
+                .product(name: "OpenAPIRuntime", package: "swift-openapi-runtime"),
+                .product(name: "OpenAPIURLSession", package: "swift-openapi-urlsession")
+            ],
+            path: "Sources/\(DiscogsService.target)",
+            plugins: [
+                .plugin(name: "OpenAPIGenerator", package: "swift-openapi-generator")
+            ]
         ),
         .testTarget(
             name: DiscogsService.test,