Guide for developing form components for apps

## Description
As more and more apps are created and used, the need for more (possibly custom) components arises.
Currently the only way to add new custom components is to create a PR to our codebase with the new component. In order to facilitate this, we should create a guide with how to create new form components that the apps can use.

## Considerations
Guide should include descriptions and examples, as well as possibly diagrams explaining some of the architecture of app frontend, to facilitate creating new components. This will also be useful for onboarding new resources.

## Acceptance criteria
- Guide is available for developers to use.

## Tasks
- [ ] Create guide

## Definition of done
Verify that this issue meets [DoD](https://confluence.brreg.no/display/T3KP/Definition+of+Done#DefinitionofDone-DoD%E2%80%93utvikling) (Only for project members) before closing.

- [ ] Documentation is updated (if relevant)
- [ ] Technical documentation (docs.altinn.studio)
- [ ] User documentation (altinn.github.io/docs)
- [ ] QA
- [ ] All tasks in this userstory are closed (i.e. remaining tasks are moved to other user stories or marked obsolete)

No files or tests are named. Start by reviewing the app frontend architecture and the existing form-component development process, then document how developers create custom form components, including examples and any useful diagrams. Done means the guide is available for developers and the relevant documentation update is complete.

Written by the indexing model from the issue text.