doc: Add a document about how charts are published

doc: Add a document about how charts are published

I did not know about the charts.gitlab.io repository. Creating a tag in the Charts codebase on Canonical is not enough to get Helm charts to users, because we tell users to get their charts from https://charts.gitlab.io using helm repo add repo-name https://charts.gitlab.io.

This document adds some information about how this final step in the publishing process works. We also need to add some more monitoring so that RMs check that this works. Currently, we only check whether the tag pipeline for Charts was successful on dev.gitlab.org, but this is not enough. We need to check whether the triggered pipeline was successful.

I opened gitlab-com/gl-infra/delivery#21908 (closed) to discuss this.