docs: add rdataframe user guide

[ianna]

---

📚 The documentation for this PR will be available at https://awkward-array.readthedocs.io/en/ianna-rdataframe-user-guide-v1/ once Read the Docs has finished building 🔨

[codecov]

Codecov Report

Merging #1836 (08a9515) into main (fec5eee) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/contents/indexedarray.py	76.60% <0.00%> (-0.19%)	⬇️
src/awkward/contents/recordarray.py	83.87% <0.00%> (-0.19%)	⬇️
src/awkward/_util.py	82.21% <0.00%> (-0.18%)	⬇️
src/awkward/contents/indexedoptionarray.py	88.90% <0.00%> (-0.16%)	⬇️
src/awkward/_connect/avro.py	87.17% <0.00%> (-0.14%)	⬇️
src/awkward/types/_awkward_datashape_parser.py	47.72% <0.00%> (-0.01%)	⬇️

[agoose77]

Hey @ianna, I hope to get the C++ dependency management implemented in #1837. Once that's in, you should be able to add ROOT as a dependency for the docs.

[agoose77]

@jpivarski I've touched this PR, so it would be best if you were the reviewer.

I've switched the CI from xeus-cling to root as our C++ kernel provider (in addition to the PyROOT integration for this PR). I'm re-using the same environment spec for wheel building and consumption to mitigate the fact that we don't build a manylinux wheel.

[jpivarski]

Looks good! (I viewed the rendered docs as well.)

My only complaint is the commented-out code (below). Once that's gone, fee free to merge!

[ianna]

@jpivarski - @agoose77 argument seems convincing. If it resolves the conversation, please, merge it.