Fix dox generation for Plasma Frameworks

The API documentation generation for Plasma Framework is broken, see the url https://api.kde.org/frameworks/plasma-plasma-framework/html/index.html, this breaks API links in the Develop tutorials for example.

This probably happened because of the change from frameworks/plasma-framework to plasma/libplasma.

This MR effectively does the same in Doxygen terms.

The result (in theory) should be:

• no repeated plasma-plasma
• libplasma is no longer in https://api.kde.org/frameworks/index.html but rather in https://api.kde.org/plasma/index.html, so everything Plasma is in the same place
• this removes tiers, because the Plasma group doesn't use tiers, only frameworks does

I'll only be certain once it's merged and rebuilt, but in any case this should be a positive change that can be merged right now.