docs: remove vector_design_prototype's reference

[Saransh-cpp]

vector_design_prototype no longer exists in vector - scikit-hep/vector#272. I have commented out the reference as there are plans to bring it back - scikit-hep/vector#273 - in the future.

[codecov]

Codecov Report

Merging #1803 (963da38) into main (65f9fcd) will decrease coverage by 0.00%.
The diff coverage is n/a.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/contents/indexedarray.py	79.76% <0.00%> (-0.19%)	⬇️
src/awkward/contents/recordarray.py	83.72% <0.00%> (-0.18%)	⬇️
src/awkward/_util.py	82.36% <0.00%> (-0.18%)	⬇️
src/awkward/contents/indexedoptionarray.py	88.92% <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%)	⬇️
src/awkward/contents/content.py	77.31% <0.00%> (+0.10%)	⬆️

[agoose77]

@Saransh-cpp could you rebase onto main, and target main? :)

[Saransh-cpp]

I always end up targeting the wrong branch haha. Rebased!