[clang-tools-extra] [clang-doc] add tags to Mustache namespace template (PR #142045)

[Paul Kirth via cfe-commits]

https://github.com/ilovepi approved this pull request.

LGTM. We may want to tweak the template some more, but we should get the test in place now and follow up if we want to change the aesthetics later.

https://github.com/llvm/llvm-project/pull/142045