How to use Kirigami on other platforms

This MR does two things:

• modernizes the CMake code to be compilable with Qt5 or Qt6, easy to change to pure Qt6 once we are ready in the future, but it still explicitly only supports KF5
• mentions how to install/build Kirigami, especially important for platforms like Windows
• minor formatting adjustments in CMake

REQUIRES !311 (merged)

How to test this MR

git clone https://invent.kde.org/documentation/develop-kde-org.git
cd develop-kde-org
git switch work/thiagosueto/qt6kirigami
hugo server