Preview build strategy adjustments

[cotti]

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.