[lld] [docs] Rename LLD docs to Markdown (PR #209894)

[Yanzuo Liu via llvm-commits]

zwuis wrote:

Please update `llvm/utils/release/clear-release-notes.py` and `lld/docs/ReleaseNotesTemplate.txt` like handling `clang/docs/ReleaseNotes.rst`. Thanks!

https://github.com/llvm/llvm-project/pull/209894