Add initial documentation

What does this MR do and why?

This MR adds initial documentation to the helm chart, specifically How to enable LoadBalancer since it's easy to get it wrong.

Screenshots or screen recordings

You can see the rendered version at https://gitlab.com/gitlab-org/cloud-native/charts/gitlab-zoekt/-/blob/add-documentation/doc/load_balancer.md

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.