Streamline guidance for using external PostgreSQL third-party solution

Existing guidance for using external PostgreSQL third-party solution exists in various places:

• https://docs.gitlab.com/ee/administration/reference_architectures/3k_users.html#supported-modifications-for-lower-user-counts-ha
• https://docs.gitlab.com/ee/administration/reference_architectures/5k_users.html#praefect-ha-postgresql-third-party-solution
• Footnotes after each RA table

The issue is to explore expanding https://docs.gitlab.com/ee/administration/reference_architectures/#recommendation-notes-for-the-database-services to clarify:

• Which nodes sizes should be selected
• PG node count in different scenarios
  • colocate Praefect's database if Geo is not used
  • remove PgBouncer nodes
  • remove Consul nodes unless Prometheus auto discovery is a requirement
• Guidance for PaaS database proxy like Amazon RDS Proxy

Related Slack discussion for context (internal).