Simplify Dashboard configuration structure

[ScheererJ]

What would you like to be added:
Simplify the dashboard configuration structure based on the experience how features have evolved so far.

The integration into the gardener operator (see gardener/gardener#9583) sparked some discussion (gardener/gardener#9583 (comment)) whether the dashboard configuration could be simplified so that it is easier to understand (and configure) for people less versed in dashboard operation. It seems like the configuration structure is deeply nested with the potential for a lot new options. However, those did not seem to come to life. Furthermore, the area of container image management seems overly complicated.

Why is this needed:
With the integration of the gardener dashboard into the gardener operator it becomes easier than ever to have the dashboard running in a gardener installation. This could lead to a wider adoption than before. However, an unnecessarily complex configuration may scare new operators off. Therefore, I think it is a good idea to look at the configuration API and try to simplify it.