Update steady state solver notebook

[dweindl]

https://amici.readthedocs.io/en/latest/ExampleEquilibrationLogic.html is pretty outdated.

@plakrisenko started updating that in https://github.com/AMICI-dev/AMICI/tree/steady_state_documentation

We need to finish that.

We might want to include this overview figure https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0312148

For updating the flowchart, see also #1073 (comment).

Ideally, we'd address #2080 beforehand.