Outdated documentation for InstantiationValidation

### Description of the bug

The documentation https://docs.altinn.studio/altinn-studio/reference/logic/instantiation/#custom-validation-rules-for-instantiation is not updated after the release of v7 of app-lib-dotnet.
We need to update this so the documentation also includes how this is solved in v7+

### Steps To Reproduce

1. Create an application of v7 try to follow the docs

### Additional Information

_No response_

Start with the InstantiationValidation documentation at the linked URL and compare its custom validation guidance with the v7+ behavior of app-lib-dotnet. Update the documentation to explain the v7+ approach, then verify that the steps work when creating a v7 application.

Written by the indexing model from the issue text.