919 Add DOI support

[ddey2]

Implemented DOI support for datasets. DOIs are also available once you release a version.

You have to set the following as environment variables:

1. DATACITE_USERNAME
2. DATACITE_PASSWORD
3. DATACITE_PREFIX

Also, just so you know, all DOIs are being published to the DataCite test instance. When ready, we can enable DOI publishing to the DataCite production instance. Refer here for details.

Steps:

1. Release a version
2. Click on the checkbox in the dialog form to enable 'publishing DOI'
3. It should create a DOI for that version of dataset.

[sandeep-ps]

I have made a minor update to the related documentation and merged the latest changes from the main branch (all checks are passing). This PR should be ready for review again.