Clarify install docs for Gadi/submodules; add actionable submodule error

[rbeucher]

Summary

• A user hit ModuleNotFoundError: No module named 'access_moppy.vocabularies.CMIP6_CVs' after pip install . from a local clone. Root cause: vocabularies/CMIP6_CVs (and the other CV/table dirs) are git submodules, and a clone made without initialising them ships an empty package-data glob, so the CMOR CVs/tables go missing at runtime. Verified PyPI's published wheel already bundles this data correctly (unaffected).
• README/docs: add an NCI Gadi installation section pointing at the ACCESS-NRI conda/analysis3-latest module (no pip install needed), and document git clone --recurse-submodules / git submodule update --init --recursive for source installs.
• vocabulary_processors.py: wrap the importlib.resources.files() calls used to load CV/table data in a _vocab_files() helper so a missing-submodule ModuleNotFoundError gets an actionable message instead of a bare traceback.

Test plan

• pixi run -e dev pytest tests/ -k "vocabulary" -q — 180 passed
• Manually loaded CMIP6Vocabulary for Amon.pr to confirm normal loading still works
• Simulated a missing submodule module path to confirm the new error message fires
• Downloaded the published PyPI wheel and confirmed vocabularies/CMIP6_CVs/ etc. are already bundled correctly

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.1%. Comparing base (2b0d546) to head (b0a8e05).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/access_moppy/vocabulary_processors.py	50.0%	9 Missing ⚠️

❌ Your patch status has failed because the patch coverage (50.0%) is below the target coverage (90.0%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #607     +/-   ##
=======================================
- Coverage   75.1%   75.1%   -0.0%     
=======================================
  Files         38      38             
  Lines       8070    8075      +5     
  Branches    1517    1517             
=======================================
+ Hits        6061    6064      +3     
- Misses      1688    1690      +2     
  Partials     321     321             

Flag	Coverage Δ
unit	75.1% <50.0%> (-<0.1%)	⬇️

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