Fixes this warning:
/srv/zulip/docs/conf.py:337: RemovedInSphinx40Warning: The app.add_stylesheet() is deprecated. Please use app.add_css_file() instead.
app.add_stylesheet('theme_overrides.css') # path relative to _static
https://www.sphinx-doc.org/en/3.x/extdev/deprecated.html
Signed-off-by: Anders Kaseorg <anders@zulip.com>
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