docs: add v3 migration doc

[TerryHowe]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.54%. Comparing base (4427369) to head (645f03b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1057      +/-   ##
==========================================
+ Coverage   82.50%   82.54%   +0.04%     
==========================================
  Files          64       64              
  Lines        4779     4779              
==========================================
+ Hits         3943     3945       +2     
+ Misses        512      511       -1     
+ Partials      324      323       -1     

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[shizhMSFT]

I think it is too early for this doc. At least, we should have an v3 alpha version first.

[TerryHowe]

Well, if we have it, we can iterate on it rather than try to figure it out after the fact.