Multicomponent (Euler, MHD) check list

This issue will be used to keep track of the implementation of the compressible Euler multicomponent equations.

- [x] Basic setup for two components in 2D (#369)
- [x] Performance improvements discussed in #369
- [x] Multiple components via additional type parameters
- [x] Adaptation of `flux_chandrashekar_stable` according to #385 (not necessarily done by @Cczernik)
- [x] Multicomponent MHD (#449)
- [ ] 3D Euler
- [ ] 3D MHD
- [x] Add entry in NEWS.md
- [x] Remove the warning that this is experimental code
- [x] Advertise it in the README.md/docs when this feature is stabilized
- [ ] Docstrings for Multicomponent MHD

Feel free to modify this list, @sloede, @Cczernik.