CI: Brain-dump the design doc

[leofang]

I've been meaning to write up a doc describing the design principles of our current CI infrastructure, which is now used in

• cuda-python (this repo)
• numba-cuda
• numba-cuda-mlir
• CuPy (WIP: CI: build cupy-cuda{12,13}x wheels on GH-hosted runners (#9794, #9974) cupy/cupy#10049, CI: fold FlexCI into ci.yml + wire .pfnci to GHA-built wheels cupy/cupy#10058)

because I encountered multiple occasions in which I want refer to it, and also based upon this doc the team has a concrete basis to brainstorm about improving and/or re-architecting the CI.

Upon completion, the current overview should just cross-reference to it.