fix: add IPython as Sphinx extension

[agoose77]

This PR fixes the root cause that motivated #1964. In fact, the problem was exclusively on RTD, but when I checked my original fix, I checked the CI build result (which has not shown this highlighting problem).

This PR adds IPython as a Sphinx extension, and reverts the changes to the code-cell language.

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📚 The documentation for this PR will be available at https://awkward-array.readthedocs.io/en/agoose77-docs-fix-syntax-highlighting/ once Read the Docs has finished building 🔨

[jpivarski]

I don't see a difference in syntax coloring between

https://awkward-array.readthedocs.io/en/main/reference/generated/ak.cartesian.html

and

https://awkward-array.readthedocs.io/en/agoose77-docs-fix-syntax-highlighting/reference/generated/ak.cartesian.html

but I guess the distinction would be in the %magic commands (IPython vs Python).

But this has my approval and can be merged when you're ready.

[jpivarski]

The last message was an hour ago, so I guess you're done with this PR. Half the tests didn't run because it's a documentation-only change. I'll press the red button and merge it.

[agoose77]

@jpivarski this PR is fixing the user guide, whose code cells are unstyled in the RTD main.