Provide REST API documentation per instance

[ledsoft]

Currently, the REST API documentation is maintained manually at SwaggerHub. However, this is quite inefficient for two reasons:

1. Manual maintenance in a separate place than the source code makes it often outdated,
2. Testing the API is difficult because different instances would require different versions on SwaggerHub.

Instead, the documentation should be a part of each deployment of TermIt so that it can be directly tested. Moreover, the documentation of the endpoints would be specified directly in code. Springdoc OpenAPI could be used for this purpose.