Add INTEGRITY platform documentation

Platform-notes page will contain detailed information:
- supported hosts and development boards
- supported Qt modules
- instructions how to build a monolith project

Task-number: QTBUG-60520
Change-Id: Ibfcf8411904fba7941b16d3140d54bbbd8e071ce
Reviewed-by: Kimmo Ollila <kimmo.ollila@qt.io>
Reviewed-by: Timo Aarnipuro <timo.aarnipuro@qt.io>
Reviewed-by: Tuukka Turunen <tuukka.turunen@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>