Categorize commands according to platform

[realSaltyFish]

There are more and more commands now. I suggest categorizing them according to the platforms they work on, for example: python3 -m joint_teapot gitea get-public-keys and python3 -m joint_teapot canvas upload-assignment-grades. Feel free to let me work on this issue or do it yourself, whichever feels better for you.

[BoYanZh]

You can do it with the help of https://typer.tiangolo.com/tutorial/subcommands/nested-subcommands/