[docs] Add theme switcher to navbar

[TimMonko]

Observed absence while setting up napari-metadata docs. Probably needs added to other napari repo docs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.66%. Comparing base (cad2c38) to head (b8735ca).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #203      +/-   ##
==========================================
- Coverage   92.98%   92.66%   -0.32%     
==========================================
  Files          14       14              
  Lines        2196     2196              
==========================================
- Hits         2042     2035       -7     
- Misses        154      161       +7     

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[melissawm]

Not sure what's up with the failures but it looks like there's no docs build in CI here so I'll trust you @TimMonko 😄