Preview build strategy adjustments

For our short-term hardening goals, we should do away with `pull_request_target` as the trigger for preview builds. `pull_request` should work for the general case, but we should confirm.

It will impact fork PRs, which won't be able to be built anymore under the current method. Since most repos in the org require a successful docs preview action run in their PR workflows, in case it's dealing with a fork PR we need to skip preview generation (but it should still be validated), and return green.

These changes need to be applied to the Vale steps as well.