Move cppdocs build to CircleCI (#19768)
Summary:
The cppdocs build job (originally run on Chronos as a cron job) was frequently broken because it was not run on every PR. This PR moves it to CircleCI and enables it on every PR, so that we can get the build failure signal much earlier.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19768
Differential Revision: D15922289
Pulled By: yf225
fbshipit-source-id: e36ef59a2e42f78b7d759ee02f2d94dc90f88fff
