[CAM-13577] Provide consistent documentation of web app configuration via config.js

Web application documentation consistently reflects the new syntax for configuration via config.js based on JavaScript modules
Guides that use the new syntax already:
- https://docs.camunda.org/manual/7.15/webapps/cockpit/extend/configuration/
Guides that need to be updated:
- https://docs.camunda.org/manual/7.15/webapps/admin/configuration/
- https://docs.camunda.org/manual/7.15/webapps/tasklist/configuration/

The migration guide covers the necessary changes for all three web apps correctly already: https://docs.camunda.org/manual/7.15/update/minor/714-to-715/#changes-to-the-webapp-config-files
We need to ensure that the referenced guides are up-to-date with the new approach in general

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