[dbm] Add supabase documentation to postgres monitoring types

[dalextorres]

What does this PR do? What is the motivation?

We're adding documentation so Supabase users have an easy guide to install the Database Monitoring product on their instances.

Merge instructions

Merge readiness:

• Ready for merge

For Datadog employees:

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/alex.torres/add-supabase-documentation/database_monitoring/setup_postgres/supabase

Modified Files

• https://docs-staging.datadoghq.com/alex.torres/add-supabase-documentation/database_monitoring/setup_postgres/

[estherk15]

Thanks for opening this PR! Created an editorial review: https://datadoghq.atlassian.net/browse/DOCS-12411