RTD: hard to understand what fails, the message doesn't include failing cause

[khsrali]

It shows many deprecation warnings that we don't need to worry about. and that makes searching for the word 'warning' difficult.

Even if you search and look carefully, you realize the actual warning message that causes the built to fail is not shown! 💁

at the moment the command is:
uv run sphinx-build -T -W --keep-going -b html -d _build/doctrees -D language=en docs/source $READTHEDOCS_OUTPUT/html