docs: add migration guide to v3

[layershifter]

This PR adds migration guide that explains breaking changes in Semantic UI React v3. Migration guide to v2 was moved to a separate page.

[github-actions]

size-limit report

Path	Size
bundle-size/dist/Button.size.js	54.04 KB (0%)
bundle-size/dist/Icon.size.js	27.35 KB (0%)
bundle-size/dist/Image.size.js	49.6 KB (0%)
bundle-size/dist/Modal.size.js	62.6 KB (0%)
bundle-size/dist/Portal.size.js	37.17 KB (0%)

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (038b306) 99.51% compared to head (0e16622) 99.51%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4431   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files         186      186           
  Lines        3511     3511           
=======================================
  Hits         3494     3494           
  Misses         17       17           

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