Kubegres `.spec.containerSecurityContext` not documented

[makkes]

The field introduced in #178 is not documented at https://www.kubegres.io/doc/properties-explained.html.

[jdblack]

It looks like the site is at https://github.com/reactive-tech/kubegres-website . I might be able to help out with a PR to the docs once I have a solid understanding of hte operator.