Update Contributing/Add new component section

[standeren]

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.

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