[clang] [docs][clang-format] Rename clang-format docs *.rst -> *.md, update refs (PR #211397)

[Reid Kleckner via cfe-commits]

rnk wrote:

Thanks for the review! I will wait to land this until the second PR in the stack is approved, #211398 , so please take a look at that when you have a chance.

https://github.com/llvm/llvm-project/pull/211397