add KDecoration 3 porting guide

Xaver Huglrequested to merge
work/cherry-pick-fef6766b into Plasma/6.3

(cherry picked from commit fef6766b)

f741b396 add KDecoration 3 porting guide 9beb31f4 Apply 1 suggestion(s) to 1 file(s)

Co-authored-by: Xaver Hugl xaver.hugl@gmail.com

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