docs: Add CRXJS migration docs

[aklinker1]

Overview

Adds migration guide for people coming over from CRXJS.

Manual Testing

Read through the migration notes and let me know if anything is confusing: https://deploy-preview-1430--creative-fairy-df92c4.netlify.app/guide/resources/migrate.html#crxjs

Related Issue

This PR closes #1405

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commit: 2f2cf76

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.11%. Comparing base (342057b) to head (2f2cf76).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1430      +/-   ##
==========================================
- Coverage   81.34%   81.11%   -0.24%     
==========================================
  Files         128      128              
  Lines        6284     6284              
  Branches     1069     1066       -3     
==========================================
- Hits         5112     5097      -15     
- Misses       1157     1172      +15     
  Partials       15       15              

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