[clang-tools-extra] [clang-doc] Add Mustache HTML output to namespace test (PR #169107)

[Paul Kirth via cfe-commits]

ilovepi wrote:

I'm not concerned at all about the history. Just wondering how much value it brings to the table to do this incrementally. I'm mostly fine with this, or with us switching mustache to be the default, and making the old implementation the `legacy` one until we're ready to delete it altogether.

https://github.com/llvm/llvm-project/pull/169107