STACKITLB-1981 | fix: NLB backend nic security group reference and docs

[david-mey-STACKIT]

Description

The STACKIT network load balancer has an advanced example for set up of a NLB with a security group for the targets.
This did not work because the wrong security group was used in it. Additionally the correct one did not exist in the provider yet. Hence, it will be added with this PR and the example is corrected.

Internal issue tracking: STACKITLB-1981

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)