Sidebar elements from imported repositories

We have some imported repositories (e.g., `tutorials`, `preeco-orga`, `openfoam-adapter`), which define their own documentation pages. We sometimes add new pages in their documentation, but we then need yet another PR here to add them to the respective sidebar, which we often forget to do (or is quite tedious).

It would be great to collect sidebar elements also from these imported repositories.

For example: in the tutorials sidebar, we could host the sidebar file in the tutorials repository and import it from there.

Another example: we have the OpenFOAM adapter in the docs sidebar. We could host a YAML file in each adapter repository, and stitch all of them together, in alphabetical order, in the tutorials sidebar file.

A solution could be with jinja2 templates and a Python script that stitches together different sidebar yaml files.

Edit: I asked in the [Hugo forum](https://discourse.gohugo.io/t/collect-sidebar-elements-from-modules/57445)