Adding examples: 2D advection-diffusion

Hi!

*ClimaCore.jl* is an amazing library for me as a researcher interested in computational fluid mechanics and high performance computing. I am now also using another Julia package *Trixi.jl* as a tool to solve problems in CFD.

As for this reason, I am wondering if I can contribute a new example case to this library. After browsering `examples`, I am thinking of adding an advection-diffusion case in the 2D form. I am also willing to establish the documentation page for the steps I go through.

Hey @charleskawczynski! I also saw that there is an another issue there about the similiar topic #2234, I am also glad to join the discussion in that issue as well! Please let me know if this would become a welcome contribution, thank you!

Best,
Yiyue Feng