Device Creation documentation updates on 6.x

Why
Spike QTBUG-840 find the areas needing documentation updates by product structure from Device Creation (DC) products and from Platform organization components.

What
Target is to update the documentation to match the identified needs, so that life of a community and customer developers would be easier with up to date documentation when using Qt 6.x

Expectations
This epic collects the stories and tasks that function as work items for the organization to improve the documentation as a team, document engineers together with developers and experts.

Result-wise it's expected that documentation matches the implementation and due to that less questions or bugs are raised by the developers from the documentation than from Qt 5.12.x and 5.15.x previous LTS branch.

Time-wise this epic should address to schedule of Qt 6.0 (2H2020) and 6.1 (1H2021).