Only build website if `commithash == latest_comit` (or something)

Only build website in CI if `commithash == latest_comit`. We have a perpetual stumbling problem where builds collide when multiple PRs are pushed around the same time.

* One fix for this is checking that the current commit hash equals the latest one (@henryleberre's good idea).
* Another fix is to make build docs. a chron job rather than on push/pull (also @henryleberre's idea)