adding sphinx_rtd_theme to RTD build to fix build

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As of recently, docs started failing:

https://readthedocs.org/projects/llvmlite/builds/22167903/

The error was:

Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/llvmlite/envs/latest/lib/python3.11/site-packages/sphinx/config.py", line 358, in eval_config_file
    exec(code, namespace)  # NoQA: S102
    ^^^^^^^^^^^^^^^^^^^^^
  File "/home/docs/checkouts/readthedocs.org/user_builds/llvmlite/checkouts/latest/docs/source/conf.py", line 20, in <module>
    import sphinx_rtd_theme
ModuleNotFoundError: No module named 'sphinx_rtd_theme'

This fixes that.

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@sklam thank you for the review!