Revert "Remove documentation presubmit action now that the RTD presubmit is enabled. (#3480)" (#3497)
This (partially) reverts commit 68dcbdd1189cd938bf6023e4e1efaf64c71629aa.
@hawkinsp points out this was running doctest, which the RTD presubmit doesn't do AFAIK. We still don't build the docs here, as the RTD presubmit takes care of that.
Co-authored-by: Stephan Hoyer <shoyer@google.com>