CMS - Expose "private" collections and ZIMs in a "kiwix-serve" and for download

With the deployment of the CMS, we now have:
- https://dev.library.kiwix.org which exposes **all** ZIMs in CMS staging area
- https://library.kiwix.org which exposes Kiwix CMS collection (or will in coming days more exactly)

We do have other collections which are not exposed in a library: branded_apps, bsf, bard, endless. These were exposed in https://dev.library.kiwix.org but this is not straightforward anymore, and I'm not convinced it is the right design anymore.

We also do have https://clients.library.kiwix.org which does expose few individual private ZIMs or private collections in a pretty adhoc manner. This involves editing a YAML file which is pretty error-prone. Both at edit time but also in term of maintenance (ZIMs gets deleted / moved and configuration becomes broken).

I feel like it is time to discuss the need again around these private collections and ZIMs and maybe (or not) identify a common and convenient solution now that we have the CMS in place.

Depending on target design we might need to modify CMS or not, hence the question in kiwix/operation at first.

Start by reviewing how dev.library.kiwix.org, library.kiwix.org, and clients.library.kiwix.org currently expose collections and ZIMs, including the YAML configuration mentioned for private content. Compare the existing CMS and kiwix-serve arrangements, then document an agreed common design whose configuration remains valid when ZIMs are deleted or moved.

Written by the indexing model from the issue text.