Implemented the missing Database endpoints (#8)

This PR contains the work done to declare the missing, non-GET endpoints of the *Database* section in the [Discogs API documentation](https://www.discogs.com/developers#page:database) into the `OpenAPI` specification document.

Furthermore, documentation extensions have been defined for the `APIProtocol` and `Client` types in the `DocC` documentation catalog to match the categories of these endpoint calls to the categories in the Discogs documentation.

Reviewed-on: #8
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/Extensions/Client.md

@@ -0,0 +1,27 @@
+# ``Client``
+
+## Topics
+
+### Initializers
+
+- ``Client/init(serverURL:configuration:transport:middlewares:)``
+
+### Service endpoints
+
+- ``Client/getService(_:)``
+
+### Database endpoints
+
+- ``Client/searchDatabase(_:)``
+- ``Client/getArtist(_:)``
+- ``Client/getArtistReleases(_:)``
+- ``Client/getLabel(_:)``
+- ``Client/getLabelReleases(_:)``
+- ``Client/getMaster(_:)``
+- ``Client/getMasterVersions(_:)``
+- ``Client/getRelease(_:)``
+- ``Client/getReleaseRating(_:)``
+- ``Client/getReleaseRatingByUser(_:)``
+- ``Client/putReleaseRatingByUser(_:)``
+- ``Client/deleteReleaseRatingByUser(_:)``
+- ``Client/getReleaseStats(_:)``