[clang-tools-extra] [clang-doc] Add a Mustache Markdown generator (PR #177221)

[Erick Velez via cfe-commits]

evelez7 wrote:

> ```
> # | /home/gha/actions-runner/_work/llvm-project/llvm-project/clang-tools-extra/test/clang-doc/namespace.cpp:261:31: error: MD-MUSTACHE-PRIMARY-INDEX: expected string not found in input
>   # | // MD-MUSTACHE-PRIMARY-INDEX: * [ClassInPrimaryNamespace](ClassInPrimaryNamespace.md)
>   # |                               ^
>   # | <stdin>:13:11: note: scanning from here
>   # | ## Records
>   # |           ^
>   # | <stdin>:15:1: note: possible intended match here
>   # | * [ClassInPrimaryNamespace](../../PrimaryNamespace/ClassInPrimaryNamespace.md)
> ```
> 
> Seems like a few more are still missing to satisfy CI.

The more I fix these, the more it seems wrong. I'm following the way that the current MD generator creates these links, yet the relative paths generated here are different than those generated in the original's tests. I need to circle back around see what's up.

https://github.com/llvm/llvm-project/pull/177221