Add example database config for main and ci

In MR gitlab-org/charts/gitlab!3438 (merged), we added explicit main and ci sections to the database configuration for Gitlab Charts. This issue is about doing the same for /etc/gitlab/gitlab.rb file:

# gitlab_rails['main']['db_host'] = 'main-pg-host'
# gitlab_rails['ci']['db_host'] = 'ci-pg-host'

The values in gitlab_rails['db_*'] will still considered default values for both connections.