Document three-step migration process for Helm chart installations to use the container registry metadata database

Context

Currently, it's possible to enable new installations of GitLab deployed using helm to use a database to store container registry metadata.

Problem

In Document one-step approach for migrating existi... (#436406 - closed), we documented the 1-step approach to migrate existing data to the metadata database. However, that approach may not be suitable for large registry instances as documented here.

A three-step approach is possible, however we are currently blocked by Container Registry: allow setting database conf... (gitlab-org/charts/gitlab#5292 - closed).

Solution

Once Container Registry: allow setting database conf... (gitlab-org/charts/gitlab#5292 - closed) is implemented, we can document the three-step approach.

Alternatively, we could also wait for Container Registry: Create import jobs for meta... (gitlab-org/charts/gitlab#5293) and document the automated (or semi-automated?) process.