Docs on instantiation under app/development/logic is outdated

### Description of the bug

https://docs.altinn.studio/app/development/logic/instantiation/ is outdated for v7 and higher.

It is also confusing that it mentions C# prefill which is documented a different place. https://docs.altinn.studio/nb/app/development/data/prefill/custom/

Open the linked instantiation documentation and review it for v7 and higher. Compare its C# prefill references with the separate custom-prefill documentation linked in the issue. Done means the instantiation page is current and no longer confusingly documents C# prefill in the wrong place.

Written by the indexing model from the issue text.