Add series to content relationship on Series page type

[LaurenFBaily]

What?

Add 'series' as a new content relationship to Story series.

Why?

For the larger, evergreen content series (for example 'Inside our collections') users can discover series or serials which are part of that theme rather than just by individual stories.

Background

Request from Editorial/Lalita https://wellcome.slack.com/archives/C8X9YKM5X/p1709209099363639

Done when

• Editors can select another 'Story series' in the content relationship tab
• Series cards appear as listing cards on the Series page eg. https://wellcomecollection.org/series/WsSUrR8AAL3KGFPF

[pollecuttn]

Size: medium

[rcantin-w]

Investigating this request;

So the articles themselves are where the content relationship exists (e.g. The Migraine Art Competition's Content Relationship tab lists "Inside our collection" as a related Series). The series page itself will then just list whichever article has it listed as a content relationship.

So to make it possible for a Series to list a Series, we need to;

• Add "Series" as a possible Content Relationship for the Series Content type
• Tweak fetchSeries function to accept params.
• Add a query to go through all Series content type and replicate what we currently do for articles (filter by the ones that have a content relationship with the Series we're displaying)
• Decide on how to display them. All articles would be in a group as a result of one query, and all series in another as a result of a second query, they wouldn't be mixed. We could probably code in a merge of the two that are then ordered by date or something like that, though.

So very doable, but slightly faffy in that I'd say to wait until we're done messing with the Prismic Model as part of the Slice Machine work.