[llvm] [llvm][Docs] Show how to format Python files changed in a commit (PR #66227)

[David Spickett via llvm-commits]

DavidSpickett wrote:

We could just link to it however the example given on https://pypi.org/project/darker/ will take ages to run on llvm-project, for the reason noted in the commit message.

https://github.com/llvm/llvm-project/pull/66227