Single Mainpage.dox

This Develop issue revealed a broken link to Plasma Core that was probably broken ages ago, it's a Doxygen naming conflict™️ because KSvg had a page named core and starts with K, which comes before P.

This is because at some point this was part of plasma-framework, where the declarativeimports/ folder was home to Plasma Core if I'm not mistaken. So keeping the dox file was probably an accident. Plus, with no \mainpage, the KSvg API index was the readme, which should also be updated at some point.

This library currently doesn't need more than a single Mainpage.dox for its docs. I'll make the documentation for this library nicer in a future MR.

After this is merged, the normal Plasma Core link should be accessible.