[Layout] Provide template snippets for Bootstrap placeholders & general comments on website load time

Description of the problem

The OEP should have a modern and reactive user interface. It lacks good loading screens for different parts of the OEP website.
This is partly due to the fact that most OEP pages are implemented as one large process that must be executed as soon as a user accesses any page of the OEP. In some cases, these tasks are above the optimal < 2 sec. and can even reach load times of >= 3 min. or even hours when uploading data.

We can optimise many things in the backend, but there will always be at least one loading time of > 10 seconds in some cases. In this case, we can use page skeletons to make the loading behaviour of the oep web pages clearer and improve the user experience.

For other cases, we should provide designated loading screens that offer a transparent view on the progress.

Possible solutions

It would be great if we had a small library of template snippets that we can use to quickly arrange a placeholder for a particular page layout using bootstrap5 placeholder components.

There should also be a place in the documentation that explains how to install and update the scss and how to use certain things like the placeholder templates.

Workflow checklist

• I am aware of the workflow in CONTRIBUTING.md