This reverts a limited part of commit ad2da2d27a
that deleted .pro files for snippets compilation.
Some .pro files which contain snippets used in the documentation
itself should be restored.
Task-number: QTBUG-90483
Change-Id: I1b03833c8ff17b5fca43a5b6c5417e8545b1711b
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>