Rework README
This MR:
- Links to tutorial, API and gallery
- Changes build commands to only cmake
- Provides basic instructions for building and linking Kirigami
- Makes other minor language and clarity adjustments
I did not test the Android instructions yet, so most changes done there are simply explanatory paths + make -> cmake stuff.
Ideally we should also fix examples/staticcmake
and provide instructions on how to link Kirigami statically, but that can be left for later.