Document steps on how to upgrade to new decomposition database yml syntax

We are requiring main key in config/database.yml. We are currently warning:

WARNING: This installation of GitLab uses a deprecated syntax for 'config/database.yml'. The support for this syntax will be removed in 15.0. More information can be found here: https://gitlab.com/gitlab-org/gitlab/-/issues/338182

Proposal

Currently the steps to upgrade config/database.yml to use main key is not complete.

1. Move https://docs.gitlab.com/ee/development/database/multiple_databases.html#configdatabaseyml to another place (not development guide)
2. Write down steps on how to upgrade:
   1. Source installation - In https://docs.gitlab.com/ee/update/upgrading_from_source.html#9-update-configuration-files, mention database.yml.postgresql
   2. GitLab Charts - no other action needed, just upgrade
   3. Omnibus - no other action needed, just upgrade
   4. Docker - no other action needed, just upgrade

/cc @ayufan @axil @DylanGriffith @twk3