Document JSON API extras

[simonw]

Summary

• document the available ?_extra= parameters for table JSON responses, including metadata, facets, and diagnostics
• note the HTML-focused extras that are not currently JSON-serializable and may raise errors
• document the row detail ?_extras=foreign_key_tables option and the shape of row responses

Testing

• not run (documentation changes only)

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https://chatgpt.com/s/cd_68e6a5eb561c8191b5a5188ce52902d6

[simonw]

Needs a few edits, but this is a good start.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.40%. Comparing base (909448f) to head (8ada534).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2504      +/-   ##
==========================================
- Coverage   90.43%   90.40%   -0.04%     
==========================================
  Files          41       41              
  Lines        6485     6485              
==========================================
- Hits         5865     5863       -2     
- Misses        620      622       +2     

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