Doc: update example configuration & external-database README

Update example-config.yaml to include Redis. Also add commentation on each chart, especially Omnibus's components in use.

Update doc/external-database.md for recent changes.

• migrations chart replaces all need for omnibus when using redis chart and external psql
• migrations chart replaces gitlab_rails functionality inside the VM, so set gitlab_rails['enable'] = false in gitlab.rb.
• Expand comments for clarity:
  • We want to provide explicit configuration, so don't provide EXTERNAL_URL env when installing
  • We're not aiming to provide PGHA in this README