Doc: updated the instructions for writing documentation

Only Qt 5 supported for building docs from 3.3 on.
Added info on using https://tinypng.com to shrink PNG files.

Change-Id: I450c0cf34e34abc54511fef93e8e5caac5ba89ac
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Doc: package signing process for Android apps

Task-number: QTCREATORBUG-13429
Change-Id: I4c18c8605f45f84a1b8f5d49c2f3056158e07435
Reviewed-by: BogDan Vatra <bogdan@kde.org>