DBM Self-hosted Postgres Restructure

[domalessi]

What does this PR do? What is the motivation?

Part of larger effort to consolidate Integrations docs and DBM docs. Starting with Postgres, and starting with Self-hosted Postgres, in particular.

Merge instructions

• [ ] Please merge after reviewing

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/domalessi/dbm-restructure-self-hosted/database_monitoring/setup_postgres/selfhosted
• https://docs-staging.datadoghq.com/domalessi/dbm-restructure-self-hosted/database_monitoring/setup_postgres/troubleshooting