Start a docs directory for the newer loader/gridding path

[charles-turner-bot]

Summary

• add a real docs/ directory to the repo
• document the newer snake_case grid_cruise(...) path
• document the new GLODAP CSV resolution/cache/download behaviour
• add migration notes for the shift away from the older MATLAB/MAT-heavy path
• link the new docs entrypoints from README.md

Notes

• This PR is intentionally lightweight: plain Markdown first, no docs toolchain yet.
• It is effectively stacked on top of Make grid_cruise use a portable cached GLODAP CSV #11 in terms of subject matter, because the new docs describe the portability/cache changes introduced there.
• I could not make this a clean stacked upstream PR branch because charles-turner-bot does not have permission to push helper branches into the upstream repo.
• So this PR targets main, but the cleanest review order is still: Make grid_cruise use a portable cached GLODAP CSV #11 first, then this one.

Docs added

• docs/README.md
• docs/getting-started.md
• docs/data-sources.md
• docs/api/grid_cruise.md
• docs/migration-notes.md

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/data_loading.jl	97.29%	1 Missing ⚠️
src/pipelines.jl	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (94.73%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.
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@@            Coverage Diff            @@
##            main      #12      +/-   ##
=========================================
+ Coverage   7.82%   15.73%   +7.91%     
=========================================
  Files         16       16              
  Lines       1023     1061      +38     
=========================================
+ Hits          80      167      +87     
+ Misses       943      894      -49     

Files with missing lines	Coverage Δ
src/DIVA_WrapperFunctions.jl	0.00% <ø> (ø)
src/GLODAP_Section_Gridder.jl	100.00% <ø> (ø)
src/correlation_lengths.jl	0.00% <ø> (ø)
src/data_inspection.jl	14.00% <ø> (+14.00%)	⬆️
src/distances_scales.jl	91.66% <ø> (+41.66%)	⬆️
src/exception_handling.jl	0.00% <ø> (ø)
src/output_fields.jl	0.00% <ø> (ø)
src/partial_cruises.jl	0.00% <ø> (ø)
src/structs.jl	100.00% <ø> (ø)
src/utils.jl	0.00% <ø> (ø)
... and 2 more

... and 1 file with indirect coverage changes

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