Add documentation on how to use custom image locations for our Helm charts

[lfrancke]

We mention this in the release notes of 25.3 but other than that we "only" have documentation on how to use custom product images.

We should add a page similar to the one for product images that explains how to use custom operator images (or image locations) when using the Helm charts.

Exploratory: Is it possible to generate a documentation reference page for Helm charts? To easily see all options that can be set using values.yaml