[llvm] [docs] Rename LangRef.{rst|md} (PR #201974)

[Reid Kleckner via llvm-commits]

rnk wrote:

Docs build passed on the rewrite PR, so I'm going to merge this: https://github.com/llvm/llvm-project/actions/runs/28971133690/job/85966845354?pr=201975

https://github.com/llvm/llvm-project/pull/201974