Add documentation for `IDataWriteProcessor`

## Description
I'm unable to find any documentation for the `IDataWriteProcessor`.

Logically, it probably belongs in this general area: https://docs.altinn.studio/altinn-studio/reference/logic/dataprocessing/

That said, perhaps we should provide a list of all the user-hooks we offer and how to use them?

Start by locating the IDataWriteProcessor interface and reviewing the existing data-processing documentation at https://docs.altinn.studio/altinn-studio/reference/logic/dataprocessing/. Determine whether the scope is documentation for that hook alone or a broader user-hook catalog; done means the interface's purpose and usage are documented in the appropriate general area.

Written by the indexing model from the issue text.