[clang-tools-extra] [clang-doc] Add standalone Markdown parsing library (PR #200302)

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github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

The build failed before running any tests. Detailed information about the build failure could not be automatically obtained.

Download the build's log file to see the details.

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/200302