Adapt fe-docs chart for new operations cluster

[marians]

Towards https://github.com/giantswarm/giantswarm/issues/29470

This PR attempts to make the fe-docs chart deployable on k8s 1.25

Included changes:

• Adds security details to values.yaml which are already applied in the chart
• Updates schema for values.yaml
• Adds ci values
• Sets explicit NGINX configuration for fe-docs
• Updates nginx-unprivileged to 1.25
• Sets separate liveness probe path

[marians]

FYI: I changed the CircleCI config to publish dev builds for fe-docs, too. Might revert this after testing.

[marians]

The app still fails to deploy because of mkdir() "/tmp/proxy_temp" failed (30: Read-only file system). I'll try to remove the need for a cache dir, since fe-docs is all static content anyway.