Update `Contributing/Add new component` section

### Description

This issue encapsulates 3 separate cases:

1. We should update what is included the main landing page for the `Contributing` section. Today it is a link to the contributing README in altinn-studio repo. This repo points back to docs, but to a different place which summarizes how to contribute to altinn more in general.

2. We should update the section on `How to create a new component` in the app. The part that describes adding an existing app-component to Studio is updated. Currently the section on _how to create_ is only a TODO. However there is a section in the old and more general `Contributing` section - but this section also needs modification and improvements.

3. The last point in the section of `Adding a component to Studio` might also need some attention as it is currently very general.

### Additional Information

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### Tasks

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### Acceptance Criterias

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Start by locating the main Contributing landing page, the app's “How to create a new component” section, and the “Adding a component to Studio” section. Compare them with the old general Contributing section and the contributing README link; done means the three areas provide accurate, consistent guidance without the current TODO or overly general instructions.

Written by the indexing model from the issue text.