[llvm] [Github] Force files changed step in docs action to use GH API (PR #69763)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Looks like this needs some work as this only works on PRs and not against `main` (which still requires history). Will need to figure out some proper mechanism to get this to work.

https://github.com/llvm/llvm-project/pull/69763