[llvm] [Docs] Update contribution guide to remove the single-commit requirement (PR #212031)

[via llvm-commits]

zhangweize9-cyber wrote:

If you are satisfied with these changes, I suggest using `git rebase -i` to `pick` the single commit message you prefer and `squash` the rest into it; then, use `git push -f` to force-push the result to your branch. I think this would make the commit history look much cleaner. Or do you have other considerations?

https://github.com/llvm/llvm-project/pull/212031