Turn Sphinx warnings into errors in CI

[yut23]

PR summary

PR motivation

PR checklist

• test suite needs to be run on this PR
• this PR will change answers in the test suite to more than roundoff level
• all newly-added functions have docstrings as per the coding conventions
• the CHANGES file has been updated, if appropriate
• if appropriate, this change is described in the docs

[yut23]

For reference: Sphinx counts warnings through the logging framework, and doesn't distinguish between log levels, so errors and warnings are treated the same. The "Malformed table" and "line-length-limit" errors come from docutils.