develop.kde.org place for the porting guide to live
Right now we have https://invent.kde.org/frameworks/kirigami/-/wikis/Porting-applications-to-KF6-version-of-Kirigami, which is an internal wiki, and doesn't seem super suitable for pointing 3rd-party devs at. We should create some external-developer-focused documentation for this on develop.kde.org. We've got this going for the Plasma widget changes that need 3rd-party devs to adapt their code and I think it's a bit friendlier and more actionable as a result; see https://develop.kde.org/docs/plasma/widget/porting_kf6. It could live at https://develop.kde.org/docs/getting-started/kirigami/.