[clang-tools-extra] [llvm] [clang-doc] Add standalone Markdown parsing library (PR #202991)

[Paul Kirth via llvm-commits]

ilovepi wrote:

I notice this PR is growing substantially in size. I think it would be a bit better to split out the base changes and add them and testing incrementally. as it is, this is growing hard to provide good feedback about, and I'm concerned more features are creeping in rather than gettign reviewed on their own.  To be specific, I'm not sure the fuzzer changes belong her and not in its own pr done later.

https://github.com/llvm/llvm-project/pull/202991