docs: Add section on extension API entrypoint limitations

[aklinker1]

Overview

This closes #1569. There was already a section about not using APIs outside the main function, but I've added a whole new section with examples.

"... you should avoid using browser or extension APIs outside the main function of your entrypoints!"
– https://wxt.dev/guide/essentials/config/entrypoint-loaders.html

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.97%. Comparing base (1272e2e) to head (91504d5).
⚠️ Report is 34 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1743      +/-   ##
==========================================
- Coverage   80.98%   80.97%   -0.02%     
==========================================
  Files         129      129              
  Lines        6627     6627              
  Branches     1080     1082       +2     
==========================================
- Hits         5367     5366       -1     
- Misses       1249     1250       +1     
  Partials       11       11              

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