docs: v13 docs & migration guide

[mdjastrzebski]

Summary

Docs for v13-next and up-to-date migration guide.

Test plan

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.16%. Comparing base (c7f8d6c) to head (8ee1657).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1709      +/-   ##
==========================================
+ Coverage   95.12%   95.16%   +0.03%     
==========================================
  Files          98       98              
  Lines        5603     5603              
  Branches      935      937       +2     
==========================================
+ Hits         5330     5332       +2     
+ Misses        273      271       -2     

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