Design doc: Add note about on-disk locations of point metadata attributes

[keller-mark]

This PR clarifies how to access the attributes listed here when accessing the points data from the on-disk format.

Is this correct that the coordinate transformations are in "transform" in-memory but "coordinateTransformations" on-disk?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.20%. Comparing base (7604a3d) to head (73b0c76).
⚠️ Report is 34 commits behind head on main.

Additional details and impacted files

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+ Coverage   92.14%   92.20%   +0.06%     
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  Files          48       49       +1     
  Lines        7473     7560      +87     
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+ Hits         6886     6971      +85     
- Misses        587      589       +2     

see 41 files with indirect coverage changes

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[LucaMarconato]

Thanks @keller-mark for helping clarifying the design doc. I edited a bit and I will merge.

Now that Zarr v3 is supported the location of where coordinate transformations are saved on disk changed a bit (it's inside a ome group). Furthermore the latest version of the NGFF coordinate transformations RFC introduces extra changes in how the transformation metadata should be written (we plan to introduce these changes soon). So I replaced your edit regarding where coordinate transformations are stored on disk by saying that we will follow the NGFF specs.