Fix pull docs build running with a schedule and increase cpp doc timeout to 4h (#88589)
* After https://github.com/pytorch/pytorch/pull/88373, pull workflow can now be triggered with a schedule. This changes the assumption in the doc build workflow when schedule event is used to determine if the docs should be pushed
* I'll create a follow-up issue to see if it's possible to improve the performance of cpp doc build job. At the moment, it uses a linux.12xlarge runner and still couldn't finish the job after 3h
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88589
Approved by: https://github.com/seemethere, https://github.com/ZainRizvi
