Documentation warnings

>  ```
> data-collections-API/docs/source/api/data_collections_api.rst:7: WARNING: toctree contains reference to nonexisting document 'api/data_collections_api.cli' [toc.not_readable]
> ```
> 
>  _Originally posted by @ElliottKasoar in > https://github.com/PSDI-UK/data-collections-API/pull/24#discussion_r2425871123_