Document the Reference — REST APIs

[zubeydecivelek]

https://github.com/CERNDocumentServer/docs-cds-rdm/blob/main/docs/reference/reference.md

Add documentation that describes the CDS REST APIs available to users/developers (authentication, main endpoints, and usage examples.

Check invenio-rdm docs:
https://inveniordm.docs.cern.ch/reference/rest_api_index/
https://inveniosoftware.github.io/invenio-openapi/