Add docs about loading the ipywidgets extension

[philippjfr]

Fixes #2594

[codecov]

Codecov Report

Merging #2602 (8b2381a) into master (a389d28) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2602      +/-   ##
==========================================
- Coverage   83.08%   83.07%   -0.01%     
==========================================
  Files         184      184              
  Lines       23296    23295       -1     
==========================================
- Hits        19355    19352       -3     
- Misses       3941     3943       +2     

Impacted Files	Coverage Δ
panel/io/reload.py	66.29% <0.00%> (-2.25%)	⬇️
panel/tests/pane/test_plotly.py	98.10% <0.00%> (-0.64%)	⬇️
panel/pane/idom.py	27.40% <0.00%> (+0.20%)	⬆️

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