Document migration from single database to decomposed for chart-based installations

Problem to solve

We need to document how administrators can migrate from single database to decomposed

We do have documentation for configuring second database. But we do not have documentation for the data migration.

High level overview:

1. Stop GitLab and Background Migrations
2. Create new gitlabhq_production_ci database
3. Copy data from gitlabhq_production to gitlabhq_production_ci database.
4. Change database configuration
5. Lock writes, Start GitLab and truncate data

Intended audience is administrators responsible for running a GitLab instance (self-managed)

Open Question

• Is this procedure the same as for Operator install?

Other links/references

• Documentation for self-compiled installation and Linux Package: https://docs.gitlab.com/ee/administration/postgresql/multiple_databases.html

Definition of done

• Document the steps
• Install a single-db GitLab instance using Helm Chart
• Following the steps should result in a GitLab instance running on two databases (main and ci)