docs: import awkward._v2 now raises an explanatory error message.

[jpivarski]

This is a follow-up of #1771 (comment).

If you think this can be handled better, let me know here. Otherwise, this is how we'll retire the use of awkward._v2.

Cc: @agoose77, @douglasdavis, @martindurant, @alexander-held, @henryiii

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📚 The documentation for this PR will be available at https://awkward-array.readthedocs.io/en/jpivarski-v2-ModuleNotFoundError-message/ once Read the Docs has finished building 🔨

[jpivarski]

Oh, this is an open call for reviewers. I'll leave this open until our regular meeting on Thursday.

[codecov]

Codecov Report

Merging #1857 (45839f3) into main (87049ac) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

❗ Current head 45839f3 differs from pull request most recent head c92ec80. Consider uploading reports for the commit c92ec80 to get more accurate results

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/__init__.py	96.96% <ø> (-0.09%)	⬇️
src/awkward/_v2.py	0.00% <0.00%> (-100.00%)	⬇️
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%)	⬇️

[alexander-held]

I think this is a great solution! I gave the changes a try and everything works as expected.

[douglasdavis]

+1