Support for setting a description of an instance that's visible in Altinn inbox

### Description

I would like support for adding a description of an instance that is visible in the inbox, when a user expands an item in the inbox. Like this:


As per now we can add presentationFields/presentationTexts to add text to the title, but I would like the ability to specify a more detailed description of what the form (skjema) is about.

Because this does not look good 😄 :


Current Altinn docs: https://docs.altinn.studio//nb/altinn-studio/reference/configuration/messagebox/presentationfields/
See related Slack-conservation here: https://altinn.slack.com/archives/C02EJ9HKQA3/p1729780951210789

### Additional Information

_No response_

Start with the linked presentationFields/presentationTexts documentation and the existing messagebox presentation configuration. Trace how presentation data reaches the expanded Altinn inbox item, then define the configuration and rendering changes needed for a description to appear there; done means the description is visible in the inbox without relying on the title.

Written by the indexing model from the issue text.