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

[Erick Velez via cfe-commits]

evelez7 wrote:

I think once I'm done adding these tests we can switch pretty comfortably. I'd just like to switch without regressing on the output, or at least documenting what's there and what's not since that's been a problem with clang-doc.

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