Add manual documentation to usim subpages

[maxfischer2781]

Short description of the feature
The usim documentation introduced in #58 is organised into topics, but mostly only includes the raw, automatic object documentation. Manual documentation on how objects can be used still needs to be added.

Describe the solution you'd like
We should manually improve each subtopic with additional information. Since topics are distinct, it would be fine to have separate PRs for each topic.

See also issue #36, which can be solved along the way.