docs: Enable sphinx_rtd_theme as an extension.
docs: Enable sphinx_rtd_theme as an extension.

This has no effect at present, but it’s documented as necessary to
enable localization of theme strings in translated output, so maybe
it’ll be relevant some day.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

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