Add changes file for Qt 5.12.3

+ 323572c5 Consistently fill out Platform columns in Qt Modules overview
+ b1074519 Handle recursion depth errors when parsing JavaScript
+ 9c821c6d Bump version
+ 583a7483 Doc: Avoid auto-linking to WebEngine, WebSocket etc
+ adaefd7c

 Document that Qt Script is deprecated

Change-Id: I01a8b5f3193aee06161433b80356d633f428de60
Reviewed-by: Paul Wicking <paul.wicking@qt.io>