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>

This commit was merged in pull request #2.