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/
https://developers.zenodo.org/?python#rest-api

[palkerecsenyi]

Discussed with @kpsherva IRL (correct me if I'm wrong!):

• We should add a relatively simple API page to the CDS docs, basically just a link to the InvenioRDM REST API docs with a brief explanation that that's what users who need an API should use
• Explain the current steps needed to generate a user API token on the UI, although we might change this soon so keep it simple
• Additionally, document some CDS-specific features such as the CERN custom fields that aren't in the InvenioRDM docs.