Documented the release and Chromium upgrade workflows

What is the benefit? Why is this valuable?

Users will be better informed and so get a better understanding of how and when updates of Chromium are taken in. "How" stands here for workflow and not for detailed technical steps. For example, users will be informed what is considered to intake a given version of  Chromium, which releases of Qt this is applied to, how security fixes are handled, where to find which Chromium is used, etc.

What are common use cases?

• Requests to Support
• Questions to Sale
• General concerns if Qt uses the most recent Chromium version
• If Qt does not use a recent Chromium version as listed for example here , is it a reason to be concerned or even file a bug report? 

Technical information

The new documentation could be added to https://wiki.qt.io/QtWebEngine/Rebase_on_New_Chromium but in this case, we should have a reference in the docs that this page contains this kind of information.