docs: factor out Sphinx utilties

[lidavidm]

What feature or improvement would you like to see?

They could be useful in other projects.

apache/arrow-cookbook#363 (comment)

[paleolimbot]

They would be useful in nanoarrow as well! (We're still using breathe there and C++ recipes are definitely in scope)

[lidavidm]

The main thing I'm wondering is if I should also push to pypi/conda-forge, or just make it pip-installable from the repo. I'll try to get to this soon 😅