Allow for non-doi-related-identifiers

[louise-davies]

Description

Adds ability to add non-doi related identifiers. Also, adds related identifiers to the landing page and correctly filters out/handles "part" relations (as these are used by the DOI for session DOIs).

Testing instructions

Add a set up instructions describing how the reviewer should test the code

• Review code
• Check Actions build
• Review changes to test coverage

Agile board tracking

connect to #{issue number}

[louise-davies]

Need to wait as the e2e tests are failing due to it not adding the part relations - this is because the old docker image tag doesn't have that code, but we can't use the main tag as that needs the draft/publish PR to merge in to be able to use. So merge the other PR first and test this code with the up to date API

The tests I wrote were passing against the main branch of the API before the draft/publish code though