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

[Paul Kirth via cfe-commits]

ilovepi wrote:

Actually, thinking about this for another second. If our plan is to remove the old backend, do we care about test parity? Is it valuable? We already think the old output is bad, and have basically stopped adding new features to it.

Do you think its a more valuable use of time to just audit these, and swap out the current tests for mustache when we delete the other backend? a quick way to check would just to change the enum behavior when we select html to do the mustache thing instead. The diffs on all tests would probably give a clearer picture of what's changing.

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