Documentation: How could an app owner set form data that is read-only

## Description
A use case for NSM is that an application could be instantiated with fields that should be read-only. How can we ensure that these fields are indeed read-only? Do we have any similar use cases in Altinn 2?

Relevant discussion on this topic: https://altinnstudio.slack.com/archives/C01AD7D4JAY/p1608209676122000

**For now we should only document how this could be implemented by an app developer.**

## In scope
- Find a viable solution for setting data that is unchangeable as part of the instantiation of an app

## Constraints
> Constraints or requirements (technical or functional) that affects this analysis.

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## Analysis

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## Conclusion
> Short summary of the proposed solution.

## Tasks
- [ ] Is this issue labeled with a correct area label?
- [ ] QA has been done

Start with the issue description and its linked Slack discussion, then look for comparable read-only form-data use cases in Altinn 2. Done means documenting a viable way for an app developer to set unchangeable data when instantiating an app, including the relevant constraints and conclusion.

Written by the indexing model from the issue text.