WCAG: Clear instructions

## Description
Does the user always know what is expected of them? I have seen other examples that have been reprimanded for not showing how the user is able to continue. Examples of this is a deactivated "next" button or a hidden "next" button. We always give the user the ability to try to continue, but if there is more tasks left they are told to fix the errors presented on the page. This is a good way of solving this issue. But I have also seen services where dynamics are hiding parts of the form page including the "next" button. If this is the case we might be breaking WCAG guidelines.

This is a similar problem as: Altinn/altinn-studio#5347

## Acceptance criteria
- [ ] The user always have a way of finding out how to continue

## Specification tasks

- [ ] Could this be solved with guidelines explaining how to avoid this for the app developer?

No source file, test, or entry point is named; start by reviewing the described hidden or deactivated “next” behavior and the related Altinn/altinn-studio#5347 issue. Done means documenting guidance that ensures users can always find how to continue, with the acceptance criterion satisfied.

Written by the indexing model from the issue text.