Collection sidebar folders (group collections under a collapsible header)

[dennisklappe]

Problem

Once a site has more than a handful of collections, the content sidebar becomes a long flat list. It gets worse with patterns that deliberately use many small collections, for example one collection per page type so each page shows only the fields it actually uses (no empty or irrelevant fields). For non-technical clients a long flat list is hard to scan.

Idea

An optional group label on a collection. Collections that share a group value render under a collapsible folder in the admin content sidebar. Collections without a group stay inline exactly as today, so it is fully opt-in and backward compatible.

Example: a site splits its pages into "Algemene paginas", "Paginas met FAQ", and "Paginas met FAQ en beeld", all with group: "Paginas". The sidebar then shows one collapsible "Paginas" folder instead of three top-level entries.

Rough shape

• group?: string on the collection config and the seed collection schema
• persisted on the collection (a group_name column)
• the admin content sidebar groups by it (a pure grouping function, easy to unit test)
• no change to routing, the REST API, or how content is stored

Why

It keeps the "one collection per page type" approach usable without cluttering the sidebar, which is the cleanest way to avoid empty fields. Small, contained, opt-in.

I have already implemented this in my own fork and it is running in production on a client site, so I am happy to open a PR that follows the contribution guidelines if this aligns with the project's direction. Would you welcome it?

[MA2153]

Would be nice to extend this to taxonomies too

[swissky]

Implemented in this shape (plus taxonomies and plugin admin pages), with before/after from a 17-collection site: #3061.