feat: change the postgres instance docu with sna and encryption

[runowadm]

Description

relates to #1234

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)

[rubenhoenle]

Pipeline failing, the docs are generated. You'll have to adjust the go code of the resource.

Furthermore the added note in the docs is not clear to me.

To you mean something like this maybe (private preview)?

terraform-provider-stackit/docs/data-sources/postgresflex_instance.md

Line 95 in 9649bcc

- `access_scope` (String) The network access scope of the instance. This feature is in private preview. Supplying this object is only permitted for enabled accounts. If your account does not have access, the request will be rejected. Possible values are: `PUBLIC`, `SNA`.

[rubenhoenle]

see above