[clang] [llvm] [docs] Rename selected clang docs to Markdown (PR #206181)

[Reid Kleckner via cfe-commits]

rnk wrote:

> Please update llvm/utils/release/clear-release-notes.py to make it work with the new suffix.

That appears to already be included in this PR.

https://github.com/llvm/llvm-project/pull/206181