docs: fill gaps in v5→v6 migration guide

[mizmay]

Made added to the guide to where there was something to add that could either (a) better document the specific failure mode a developer might encounter or (b) highlight something they can do to verify or address it.

• CommonJS removal: got more specific and included the error message one might encounter
• @ latest 404: what you will encounter if you have @ latest set and specifically what to do instead
• WebGL2 requirement and map.transform removal: included even though they aren't strictly migration tasks to highlight a specific action mentioned in the CHANGELOG and perhaps worth repeating here.

From #8168: deliberately not addressing the vite callout (already fixed in the vite docs so nothing to address) and the peer-range guidance for library authors (doesn't fit here).

Planning a follow-up migration skill in maplibre-agent-skills maybe just a proof of concept, but who knows, maybe it's useful to someone.

Launch Checklist

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Assisted-By:

Claude Sonnet 5.0 for research and testing to verify the failure modes, etc.

[HarelM]

THANKS! Added a minor comment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.23%. Comparing base (062d745) to head (f7d3061).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8227      +/-   ##
==========================================
- Coverage   94.23%   94.23%   -0.01%     
==========================================
  Files         291      291              
  Lines       25030    25030              
  Branches     6610     6610              
==========================================
- Hits        23587    23586       -1     
- Misses       1443     1444       +1     

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[CommanderStorm]

minor changes

[HarelM]

Feel free to squash-merge this when you see fit.