feat(docs): add postgresql backend example

[rubenhoenle]

Description

• add postgresql backend example
• also add warning about state locking for s3 backend

relates to STACKITTPR-729, STACKITTPR-738, #1534

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)