[llvm] [Github] Fetch number of commits in PR for docs action (PR #69763)

[Tom Stellard via llvm-commits]

https://github.com/tstellar approved this pull request.

LGTM. If the speed is still an issue, the other solution would be to have separate workflow files for the clang and llvm docs.

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