[clang-tools-extra] [clang-doc] integrate JSON generator with Mustache templates (PR #149006)

[Paul Kirth via cfe-commits]

ilovepi wrote:

Is it possible to do all the JSON refactoring as the first PR, and then do the mustache integration after? I believe those can be split w/o too much trouble.

https://github.com/llvm/llvm-project/pull/149006