[llvm] [llvm] Add a tool to check mustache compliance against the public spec (PR #142813)

[Paul Kirth via llvm-commits]

ilovepi wrote:

@nikic I realized the doc file wasn't following the format we use for most other tools, so I made it match more closely. I wouldn't have noticed, but the `*.json` in the command broke the docs tests, and I realized things were not quite right, so I just made it more closely match other `rst` files.

https://github.com/llvm/llvm-project/pull/142813