The Convection Release

A big one: 94 pull requests since v5.6.

• 🔥 3D Rayleigh-Bénard convection with its own problem class, project and HPC benchmarks, on top of a
  largely rewritten spectral framework with Neumann BCs, a VkFFT backend and heterogeneous CPU/GPU
  computing, thanks to @brownbaerchen (#563, #571, #593, #628, #651)
• ParaDiag for collocation methods, thanks to @brownbaerchen (#530, #577, #579)
• new StroemungsRaum project, from FEniCS heat equation examples to incompressible
  Navier-Stokes, thanks to @Ouardghi (first contribution 🎉) (#598, #619, #630, #642)
• more generic QDelta coefficients and FieldsIO fixes thanks to @tlunet (#550, #551, #584)
• CI and packaging maintenance by @pancetta, shrinking the wheel from 20.5 MB to 1.2 MB
  (#602, #611, #659, #663), I smell Claude here

⚠️ Three changes are not backward compatible, see the changelog.