[clang-tools-extra] [llvm] [clang-doc] Adds a mustache backend (PR #133161)

[Paul Kirth via cfe-commits]

ilovepi wrote:

@PeterChou1 I've uploaded a patch set w/ the relevant changes from this patch. Some of the details were split into standalone patches, but I'd appreciate if you could help both review and test those patches. It would also be nice if you could share any testing corpus or lit tests you have.

I think we can leave this PR up until we actually land something in tree.

https://github.com/llvm/llvm-project/pull/133161