docs: Update sidepanel availability

[aklinker1]

Side panels are supported by firefox, both MV2 and MV3. WXT already supports them, but it's docs were wrong.

This closes #248.

[netlify]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e80c134) 64.28% compared to head (17fb5fd) 64.28%.

❗ Current head 17fb5fd differs from pull request most recent head fa95044. Consider uploading reports for the commit fa95044 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #250   +/-   ##
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  Coverage   64.28%   64.28%           
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  Files          76       76           
  Lines        6029     6029           
  Branches      489      488    -1     
=======================================
  Hits         3876     3876           
  Misses       2116     2116           
  Partials       37       37           

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