Fix outdated documentation on subscribing to events

[acn-sbuad]

Description

Get started documentation on subscribing to events is outdated.

https://docs.altinn.studio/events/subscribe-to-events/get-started/api-access/
Should be updated to reflect current situation.

Add a link to existing documentation for how to define a client/integration in Maskinporten and ID-porten.

Tasks

• Update documentation.

Acceptance Criterias

• Up to date documentation about the subscriber scope and whom that can use it

[olebhansen]

Perhaps add a link to https://docs.altinn.studio/authentication/getting-started/maskinportenclient/ (suggest splitting this to a common page multiple products can reference)